How can I get my aunt to stick to her diet?

My aunt Tammy has become very obese lately. Every time I visit her she says she is going to start a diet, but when I come back a month or two later, she has gained 13 pounds!!! She is 53 years old and I am worried for her health. We went to go try on clothes and she didn't find one that looked good on her or fit. I felt really bad for her :( Is there any way I can help her commit, or are there any foods that are healthy and she will enjoy? Also some easy exercises?Please help, I want her to be happy!