Aurora would be good. Its small but usually needs to be a well established plant to produce numerous blooms.
Paul Hariot is another small lily that is an excellent bloomer. It might be a bit too large for a 40 gallon tub though. Helvola is a very tiny lily but I'm not sure how it does when it comes to blooms.

Helvola is probably the best mini lily, although I don't think it has a fragrance.

True 'Mini's....
I would recommend Joann Pring, Helvola (2nd watergals motion), and the mini white called Nymphaea pygmaea. (very similar to Tetragona)
They all can be grown each in a half barrel planter or similar sized container.

They all do have a very slight fragrance, but you have to stick your nose right down into them, hopefully without encountering a bee of some sort!

Day bloom tropical waterlilies Dauben, Tina, Islamorada, St Louis Gold, Ganna Walska, Lavender Lace have stunning fragrance, can do steady blooms in threes, fours and more when you get them going.

Very easy to Winter indoors

There are some excellent small hardy waterlilies however at best, fragrance is barely noticeable

Chrysantha (stunning marbling), Aurora, Little Sue, Graziella, Indiana*, Charlies Choice ~ changeables

Hermine, Walter Pagels, Paul Hariot and Perry's White Wonder ~ white

Bateau, Cherokee, Clarissa, Ellisiana, Froebeli, James Brydon*, Laydeckeri Fulgens, Laydeckeri Purpurata, Perry's Baby Red*, Perry's Dwarf Red, Perry's Little Champion, Perry's Red Beauty, Perry's Red Star, Pygmaea Rubra, Reflective Flame, Sanguinea ~ Red

Pink Sparkle (excellent marbling), Beldts Dwarf Pink, Mary Patricia, Perry's Fire Opal*, Jean Marie, Laydackeri Lilacea, Joanne Pring, Jean Marie ~ pink

Berit Strawn (stunning marbling) ~ peach

There is some room for argument as to which you might call red, hot pink or pink, colours can vary in tone through their flowering season

* Some wiggle room on size and spread, Indiana is very much a medium size waterlily five feet wide in a fertile pond but can stunt down to two in a container

Chromatella, Pink Beauty and Rose Arey are fairly easy to grow at a small size.... less than three feet wide

There are others. Solfatare and Betsy Sakata are smallish yellows

I have Chrysantha, Helvola, Joanne Pring, Hermine, Charlie's Choice, Perry's Baby Red and Mary Patricia. All are small spreading.

Perry's Baby Red is my best bloomer but it is much larger than Helvola and Joanne Pring which are also good bloomers.

I also have to vote for Joanne Pring- Very attractive dark pads, multiple small pink blooms, compact spread...

I agree. Joann Pring has multiple blooms all summer for me. :)
A LOT of the water lilies suggested above are not suitable for such a small container without a LOT of maintenance to keep them 'stunted'.
And if your dad is ailing, I don't think he is up to high maintenance water gardening.
Or do you plan to do all the maintenance yourself?

I would stick with the true dwarfs and pygmies like Joann Pring, Helvola, and Nymphaea pygmaea or N. tetragona. :D

My Joann Pring and N. pygmaea blooming even after a brutal move, and no repotting this year because we were so busy moving.
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These are growing in half barrels and VERY crowded!
The blooms are less than 2' wide. :)

Paul Hariot is not a white lily. It is one of the changeables.

This pic shows a day 1 bloom at the bottom, day 2 bloom is upper right and day 3 bloom is upper left.

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Joanne Pring is unsuitable for a small position, while its leaves and blooms are small it has a very lanky growing habit, typically three or four foot wide, likely to be a mess in less than a two foot wide space.

As Joyce's photograph indicates how scruffy a plant can be grown by someone with very limited experience, you would get nothing like that mess with Pink Sparkle or Gypsy, which have a very neat small growing habits

It would be difficult how the conclusion that stunting a waterlily requires more maintenance. Stunting a waterlily requires less maintenance, it is as simple as using a smaller pot, less dirt, less fertility.

I've had Joanne Pring planted in a mud bottomed small liner pond since 2004 and it has never grown larger than an 18" spread though it has bloomed very well. It doesn't look like a mess at all. I would recommend it for a small pond in a heartbeat.