By now, yer best bet would be to have him take it to a JD Dealer shop, and have them fix it. That way, he'll get it fixed, if possible, and the dealer will be the one who gets blamed next time, not you!
I fix things for folks, too, and i've had a few times where they have come back complaining, and it turns out they did something to it, later, but were trying to blame me, figgering i didn't charge as much as the dealer shop. WRONG!!

I looked at the belt and it is all twisted up and sheaded. Is there a right and wrong way to put on this belt?

green445---check the deck spindles and all idlers. These entry level mowers had a problem with spindles lasting. The idlers are also lighter than the premium mowers. Check to see if you can find a belt threading guide (how it goes around the idlers.) It might be in the Oper. Manual or check with your local JD dealer.

The belt threading guide is on the internet. I have it printed out here. I guess it confirms there is only one way to put the belt on, and that is what I did. Pretty straight forward.

I just took the deck off and checked the spindles. The front (middle) spindle has some play in it, not much but you can hear it is loose and feel it move. The other two have zero play. How do i know if I need to replace it? Can that little bit of play cause belt issues?

If the spindle is bad, it will be growling and will have a lot of play, especially in the lower bearing. To cause a belt issue, the spindles would have to just about to fall out, so I don't think that's the issue. Sure, he'll need new spindles eventually, but not yet it sounds like.

Check the idler alignment. I know some of those 48" deck would chew up the belt if you mow at a 2" or lower cut height for a long period. Given how long he's had this tractor w/o problems, I doubt that's the problem. Unless somehow he has bent the idler bracket. There is a flat idler (not V idler) under there where the belt exitst he deck and runs back to the clutch. This is where the problemc could be. At his usual cut height, make sure the belt (it will be the back side of the belt) is running in the middle of the idler. The idler will be at an angle, but the angle could need tweaking.

I suspect something has changed with the mower recently since he's just now having problems. You just have to figure out what.

I did notice he is cutting his grass way too short. I put new blades on it and he has them scuffed up more in one cutting than I do all summer long to my blades. I think I'll put the new belt on, check the middle idler, and return it to him. I'll say that he is cutting too short, and if he has problem with this belt then he needs to take it to a JD dealer. I'll also mention he will need a spindle soon, and probably pick up in advance. Thanks, much appreciated.

I put the new belt on. When I started the deck it jumped around and killed the engine. There is something wrong here. It's going to the dealer. I did notice that the belt is not riding in the middle of one of the idler pulleys, the one you take the bolt off of to put the belt on. It also had a little play in it.

Well back again a year later. The problem was the idler alignment was not right. The reason was because of wear on the metal busing that held it in place. We got it through last year. This year I'm putting on a deck kit and replacing the bushing on the idler.

I have one more question about the three pulleys. When you mount the pulley to the spindle, there is a hex pattern on the pulley that matches up with the same hex pattern on the spindle. I get a new spindle with the deck kit, but keep the same pulleys, and that hex pattern in the pulley is worn and almost stripped out. Do I need to replace the pulleys also, or is the bolt what really holds them in alignment?

If you are saying that the portion of spindle shaft where the pulley will rest is hex shaped, and the pulley itself has an internal hex "socket" that slips down over the hex shaft.......then yes, you need to replace the pulleys too.
The hex shape of both components is what really bears the torsional load, not the bolt.
The bolt merely pulls the pulley down snug against the hex shaft.
If the wear to the pulley is excessive, what do the old hex shafts look like? Are they on the verge of "rounding off"?

yes, they are worn to the point of rounding off. It's like putting an old worn socket on a new bolt, and it slips easy.

They look like this.

Here is a link that might be useful: Deck Pulleys

Get new pulleys to go with the deck kit.
Otherwise, it will not be long until the old pulleys wallow out, and take the hex shafts with them.
With this kind of configuration of shaft to pulley, I believe it would be prudent to buy some commercial "stud and bearing mount" compound from a bearing supply house and coat the hex portion of the shaft and pulleys as you assemble the deck parts.
The "stud and bearing mount" compound would improve the fit, and torsional grip of the shaft to the pulley (and vice versa) which could even eliminate the wear problem you are having now. It would also be wise to periodically (at least at the beginning of each cutting season) retorque the bolts that secure the pulley to the hex shaft.

How much torque do you think? The nuts for the idler pulleys are torqued to 35 ftlbs, but that does not seem like enough for these.

Determine the bolt size, and grade. Then refer to the chart for torque value.
Unless you know for a fact the bolts are Grade 8, consider them to be Grade 5.

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Thanks, nice diagram. It looks to be an M14 (9/16) so I'll go with 120 ftlbs.

This may sound silly here.............I hope you are not referring to the wrench size when you say "M14" (9/16).
If you are calling out the wrench sizes that fit the bolt head, stop right there.
Bolt size is the diameter of the bolt, not the size wrench that fits the flats.
If the bolt you have uses a 14mm (9/16") wrench, you have a 3/8" (possibly a 10mm) and as such, the torque would only be 36 to 40 foot pounds.
A 3/8" bolt will never withstand 120 foot pounds.

Good question. That is the bolt size.
It's a metric bolt, measured M14 on my bolt guide, which is closest to a 9/16.
It requires a 22MM socket, which is almost 7/8.

OK, I just wanted to make sure about that.
That exact scenario pops up often at auto parts stores when somebody arrives wanting a bolt, or nut.......and erroneously gives the wrench size as the "bolt size".
You're OK, I just had to ask.

I want to know if I'm at John Deere l130 lawn mower if one of the pulleys on the belt deck is supposed to be at an angle instead of level?

All of the pulley should spin in the same plane. OR to say it another way; using meter stick (yard stick), laid the edge across any two pulleys, both sides of both pulleys should touch the edge of the stick. All pulley should pass this test.