I that that maybe a more long term weight loss present would be better.  For sure the camp would be fun and might help a little, but don't you think that after the weekend she would return to old habits?  I don't think this is offensive for one reason: that she as reached out and asked you for help.  You can write in a card that all you want is for her to be happy.  Of course you'd love her any way she was, but you wanted to give her something she's been wanting.  I think it's a really nice idea.