hmmm. I think if she really WANTED to "get fit, toned, sexy" the motivation would be there and she would be going. Could it be that she wants reassurance from you that she doesn't NEED the gym to be everything and more that you have ever dreamed of in a life partner?
Seriously, whether or not you intend it as any kind of comment on her, can you see that by buying her the sessions, you are actually making a comment and one that might be hurtful?