i dunno, i would be sensitive about it for sure. start going to the gym and inviting her along. eventually she will go to the gym with you, and you can get her doing some exercises and tell her that they will help her keep her weight in check, she will get the idea, but its pretty subtle.
when you start going to the gym, ask her to set goals for herself, she definitely knows she has gained a lot of weight, and she will probably set her goals to lose it. then keep encouraging her and help her eat healthy.