I think that giving a “pointed” gift is, in general, an awful idea, not because it’s not well meant, but because the vast majority of people who might, for instance, benefit from more exercise already know that and don’t get round to it for some reason (either because they have a genuine physical reason why they can’t, or because they’re just not motivated, or whatever), not because they genuinely would like a gym membership but can’t afford it or didn’t think of it. So in the vast majority of cases it won’t help, so however well meant, it’s likely to come off really awfully. Many friends have problems one wishes one could fix, but you can’t always do it, and it’s often insulting to try, because while SOMETIMES your ideas are better than theirs, typically you’re suggesting something they already have thought of.
However, DDR is something that is genuinely fun, and that many people would enjoy, and many people have found actually DOES help them get fit, even though they’d not thought of it until a friend recommended it. So I think it s quite a reasonable present.
Basically there’s two questions. (1) Would you get this gift for someone else unrelated to someone you thought specifically might use more exercise? and (2) If not, if you’re specifically trying to help, is it in any way likely to ACTUALLY help? I submit 2 is almost always no, but that here 1 is probably true. Hence, I think DDR is a good idea.
It’s certainly reasonable to tell her you enjoyed it and ask if she’s ever tried it. She’s likely to say something like “no, I hate the idea” or “oh, it sounds fun, but I never have”, and you can take your cue from that.
Edit: On rereading the post, I see that she is your best friend. If so, my advice may be overly negative, and it would be reasonable to talk to ask her about it. My limited experience advice says, simply be helpful and non-judgemental. Most people (visibly overweight or not) could benefit from more exercise and know this perfectly well, and might value talking to someone about it. What people REALLY REALLY HATE is where everyone automatically assumes that it would be easy to fix if they weren’t so stupid they haven’t thought of blatantly obvious ideas, so it’s necessary for everyone else to constantly point these out…