Ahh I don't mean to be snide, but how much can an 87 year old eat?. My mom gives me money every week to buy her groceries but I find that she is eating less and less and less now while approaching 86.
You are wise to ask early. She can pay you but you need to keep good records. To be totally safe legally, I'd have an attorney draw up a contract. Then, if she goes on Medicaid, you will have the right records at your fingertips.
Carol