Leave them alone. Stay out of the way. Make them pay you for your advice.
I saw a squatter today walk forward to unrack the bar and backwards to rack it.
I almost always follow the prime directive and say nothing, but lately I've advised a few doing it rack-backwards, worried it may be dangerous.
Is this actually dangerous enough to intervene? It's often younger ladies doing this, and I'd prefer not to look like the old guy at the gym who offers to spot squats. Can I keep my mouth shut in good conscience?
Leave them alone. Stay out of the way. Make them pay you for your advice.
One point to clarify... you mention young ladies. I assume they're squatting low weights for high reps, i.e. nothing really too taxing. If so, I agree, just let it go.
If, on the other hand, they're actually lifting decent weights for 5s, i.e. something that might actually be a challenge, I don't know that my conscience would let me keep my mouth shut.
Let them get hurt. Pain is instructive. Even though the exercise will get the blame instead of the technique, young ladies have been taught that you're a creep if you mansplain to them, so there is no way to help.
Of course you tell them. Fairly easy to do without looking like a creep.
Yes, tell them. If nothing else I want to know how they respond.
A while back there were a couple of high school aged guys doing this in my gym. It was pretty obvious they were very new to the whole gym thing (as if I'm some seasoned vet). I did tell them why what they were doing is not a good idea and they said they were doing it that way so they could see the mirror (which was across the aisle and behind a leg press machine). I decided to let that part go, and just showed them how to move the hooks on the rack. They did one or two more sets and I've never seen them again... so yay, less competition for a squat rack.