When (if ever) is it appropriate to comment on someone's diet and exercise choices?

One of my favorite advice columnists posted recently about commenting on spousal weight gain. Do you think it’s appropriate to point out to your spouse that his/her habits are causing weight gain, and it bothers you? What about a boyfriend/girlfriend?

Or, what about a close friend who complains about being overweight but continues to make bad choices? Is it appropriate to say something in that case?

I was thinking about this advice column this morning as I watched an overweight woman fish a donut out of a pastry box this morning. I know it makes me a jerk to judge total strangers, but I just don’t understand what would cause someone to make a decision like that, and I wonder what would happen if the people who care about this woman said something.