1) Explain to her that in this world there are solitary fish and schooling fish. Bettas are not schooling fish.
2) The problem with tankmates is that they will either nip the betta's fins or get eaten by the betta
3) If he seems bored, tell her to
-Place new decorations in the tank eg plants or caves. She can rotate them so that they will seem new every water change.
-Move the 'furniture' around once in a while.
-Float a ping-pong ball on the water for him to flare at and push.
-Keep him near a desk or kitchen... somewhere where he can 'people-watch'.
-Place pictures or objects (even those plastic animal figures are good0 outside the tank for him to investigate. You can rotate them daily.
-You can also purchase these awesome 'betta toys' from Zoomed ---> Search Results: Products
-Give him some live food (bloodworms, blackworms, brine shrimp are great) to hunt.
4) If you must go with a tankmate, I advise against other fish and apple snails. Apple snails get pretty big and are pooping champs so they might overload a 5g and you don't want your grandma having to do any extra maintenance. You can try it but I think there are some better options:
-Ghost or Amano Shrimp
- 1 or 2 Zebra (Nerite) Snails (Highly recommend)
-Ramshorn snail (Only one or they will breed!
Be aware that even these tankmates are subject to being picked at by the betta. But at least they won't attack him!