Dating: What if they gain weight?

What would you do if the person you were seeing suddenly put on a lot of weight?  You are basically faced with the options:  being a jerk and dumping them, being a jerk and talking to them about it, or shutting up and stay supportive.

Is there another alternative?  My friend Grey said that he actually met his girlfriend in a gym.  She was super active and fit two years ago.  She had an injury that limited  her activity for a period of time.  After she was cleared to return to exercise, she turned into a couch potato.

He wants to stick it out with her but he admits that the attraction has been waning.  What is the best way to broach the subject?

If you were the one who put on some extra pounds, would you want to know if it bothered your partner?

Do you think weight gain is a legitimate reason to break up with someone?