Does that really happen? I am somewhat surprised: I know there are plenty of men who try to "hit on" women in the gym, which I can well understand is creepy, but I have never seen one pull the stunt of trying to "advise" a woman how to lift.

I used to go to a gym that had separate but identical sections for men and women but then changed to be a an ordinary mixed gym. And even though I am a man, I kind of regret it, because when the sexes trained in separate sections, people were basically "attending to business" - they were there to train. Once the gym became mixed, there was a lot more time wasted with men (not just the gym goers but also the personal trainers, etc.) trying to impress or hit on the women.