@hugh_gilly brought up a question in the "When to Give Advice" thread that was pretty relevant here and got a little lost in the shuffle there. Figured the trainers would have a good take on it.
It's a bit of a gray area because it's not 100% unsolicited advice since one of a floor trainer's jobs is to help when they can, but it's not 100% solicited because the person isn't flat-out asking for help.
This is where communication, the proper approach, and even a bit of salesmanship comes in. You never want to tell someone they're doing something "wrong" because it puts them on the defense from the start. You acknowledge what they're doing and offer an alternative and explain the added benefits from your way. Sell it, explain why it's better, but leave them the option to keep doing things their way.
Let's say someone's bouncing the bench off their chest, I'd wait until they're done with the set and say something along the lines of: "Chest day? Cool. Not a lot of people still hit the big exercises like the bench. Real quick, can I show you a tweak to get a little more chest work out of each rep? I saw you were kinda bouncing the bar off your chest, but that actually takes stress off the muscle and makes it harder on the shoulder joint. But if you pause at the chest for a half-second, it makes the muscles contract harder, so you get more pec activation and more growth. Give it a shot on this next set? I'll spot you for a minute."
Compliment sandwich. Praise-critique-praise or positive-negative-positive. Old school technique but it works.
But if an exercise choice, form, or other issue is, like 50-50 take it or leave it, make the judgment call. It's okay to not "save" everyone, because that can very quickly turn you into the Exercise Nazi Form Police. And nobody wants to hire that guy to train them. Quarter-rep squatters don't always have to be corrected. People who solely use machines and avoid free weights don't always have to be enlightened. Play it by ear in terms of who to help and when, but whenever you do help, be sure to avoid the literal phrase "you were doing that wrong."