I agree that the best thing to do is to compliment something about them that is NOT weight related.
A few years ago, I helped out in a friend's nutrition club, working behind the counter. I made a point of smiling at everyone as they entered, and, as I got to know them better, I would look for something to compliment.
This was in our WAYYYY northern suburbs, more like the rural parts of MN than the Twin Cities that they are actually part of. And people who give compliments are under suspicion, because they MUST have a nefarious intent.
But, after a while, they knew that I meant it, and it was fun, whether a member was overweight and losing, overweight and gaining, or had lost down to goal, to be able to get a smile by complimenting their hair, or their sexy lipstick or that blouse that looked so nice with their eyes. (Different types of compliments for the guys, of course!)
Laura, I'll bet if you told your friend that her outfit looked cute on her, she'd have happily talked about her weight loss, and assumed you were complimenting her on her weight!