whoa when I read the title i was gonna say something along the lines of "youre a douchebag, you shouldnt care blah blah blah" but gaining 60 lbs is crazy.
Do you weigh a lot? maybe she feels like she can get away with it.
The best way is to offer to do a "couples fitness/yoga/pilates" course together or something, like make it seem like you want to spend more time with her. I'm sure if you can find out a way to find out her weight then you can figure out a way to ask her