I would buy her a present of a betta care kit - a Kritter Keeper, a heater and a little cave, and help her adjust one of her bettas to this. Maybe when she sees the difference in his health, she'll be motivated to help the others, too.
If you can afford it, of course, you could buy 4 such things, but obviously it isn't fair that you should have to.
That was the only thing I could do for a friend who was keeping three goldies (two comets and a fantail, one comet has since died) in a 5 gallon tank. I bought her a filter, a bottle of Prime and a bottle of Stability to kickstart the cycle as an "Easter present". Hopefully, I'll also be taking her remaining comet for my pond.