Mention it or don’t? How should I phrase it?

If you say anything I'd keep it completely away from commenting directly on her looks. "Hey how are you? You look like you're hard at work! Great job on your commitment." And if she's at the gym, she's there to work out, likely has limited time, so don't waste her time. Short and simple. If she finishes working out and has time left to chat at the end she can find you to continue the conversation. If she brings up appearances at that point then polite compliments on her appearance are fine, but let her lead it that direction. You don't know how much she may or may not have had to put up with nasty comments in the past so it's better to let her open that line of conversation up if she is comfortable doing so. It's pretty safe to assume most people (not just women) have had some nasty comment on their body at some point and it affects different people differently. It would also be completely naïve to pretend like this isn't a sensitive subject for a LOT of people regardless of the current state of their appearance.

If you were good friends my opinion might be different. But I dont even comment on my best friend's body changes unless she brings it up, and only then only nice things to say no matter which way her body looks. And never to compare to how her body used to look.

I had a close friend once slap my belly and back between her hands and go "you're always so thin!". This friend is my most weight insecure friend. I hated that she did that to me (I was a pre-teen). I had never thought about my weight or eating habits before that and her doing that to me felt like her hammering those thoughts into me. I've never been as comfortable with my body since. And maybe that had to do with the time in my life that it happened. But it's been 20 years and I still think about it once in a while. Not all people like receiving compliments and you can't know which side of the scale she is on.