What can be done for my 88 year old grandmother who has stopped eating and has renal failure and congenital heart defect?

It sounds like your grandmother has made the decision for you. She has had a long life and she has some serious health issues. The best thing I can tell you is to have a talk with your grandmother and ask her what she wants to do. It may be that she has had enough of treatments and hospitals, so you can enroll her in hospice, spend some time with her, allow her to be with her family.