Peaches grow pretty dang fast. Apples sometimes grow fast for one year, then pretty slow after that. Cherries are somewhere in between. I have no experience with pears or plums. My gut and my mouth say, go with peach. Not sure of which cultivar is fastest for your climate but I bet just about any will be fine. Perhaps Contender or Elberta if you are concerned at all about the occasional spring cold snaps killing the flower buds, although in 6b it's probably not too huge a concern. Peaches need full sun and some water to keep them going. If you can't assure good watering, cherry might be the better way to go.

It would help to know the state. Some trees like apricot are much better in some climates than another. Also many fruits besides mulberry make a mess with fallen fruit if not cared for. Unless she wants fruit a pure shade tree would be a better bet.

J. plums and peaches can be nice bearing trees from whips in about 3 years if you grow them in a perfect environment.

May I ask why a fruit tree? Almost all fruit trees need some level of care otherwise; it could be a mess when diseases or insects attack the tree and/or its fruit. It's not pretty to have fruit drops everywhere or tree limbs ooze with something that could also end up killing that tree.

There are a number of beautiful flowering trees around. Would your friend consider that?

When my friends asked if they could plant a fruit tree with no care in my zone, I could not think of anything to recommend because even an easy-care, disease resistant apple tree still needs some attention. I suggest they plant a flowering tree.

As far as peaches, I guess where you live affects the growth. I am in north Georgia and planted 2 peach tree whips a year and a half ago, pruned to knee high at planting and was shocked at how fast they grew - 15 foot spread by the end of the 1st growing season. They are now at the second leaf and each tree is producing somewhere between 25 and 50 fruit.

Flowering Japanese Cherries are beautiful; so are flowering crabapples. Great flowers!

So far the most trouble free tree that I grow is Cornus mas,Cornelian Cherry.
Nothing attacks and all I've ever done is fertilize and water once in a while.From what I understand,it can take very cold temps.
It puts out beautiful yellow flowers in early Spring and it even fruits on its own,something that some sources don't agree with and the fruit makes a delicious sauce for ice cream and even meat.
Mine is in a container,but they can go in ground and grow to about twelve feet.Fast growing?Well maybe not the fastest,but they do get a lot of leaves. Brady
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you will definitely want to consider disease and pest resistance as your friend is not going to stay on top of it. For example I have a Haralred and a Honeycrisp apple trees in my back yard. without a lot of attention that honey crisp would have died each year so far. the Haralred is a trooper and would have been fine without me. Although I do the organic thing, If someone is going to spray them on schedule that helps the survival.

Apricots grow very fast, but they grow very long thin branches..and can be prone ot breakage if you don't thin...peaches grow very fast..but of course you are going to deal with a lot of fruit at some point (self pollinating). Apple seedlings or on a std rootstock should grow big/fast quickly. Pears grow fast enough, but grow in a Chrstmas tree shape.

Maples grow like weeds around here....

I'd go with oak trees...