First off, do bear in mind that she is not lonely. If you get a snail, it's for you, not for her.
If you do get a snail, I suggest a nerite. They stay smaller than mystery snails and won't breed in fresh water like ramshorns or malaysian trumpets. They eat algae, algae wafers and any feed your girl misses.
Another option is shrimp. They have a smaller bioload but some bettas will eat them.