It kinda depends on her health, doesn't it? I mean, if its something like she has to be on oxygen all the time, then it might not be realistic...
I know I used to have a dog who would play soccer with me... kinda... (I would kick the ball, and he would, um, bite it. Anyway I ran around a lot, which is the point) I got way more exercize playing with my dog than any sport I went out for, which mostly consisted of me standing around being bored. (Think Calvin and Hobbes =-) Anyway, I do think soccer's a great idea, but it certainly isn't the only one. If she doesn't want to go for soccer, mabe just to a family soccer thing in the park, or make up a game that involves lots of running around, or something.