I'm figuring this means you're a trainer there, and not front desk or janitorial?
You would be offended, and rightly so. People don't want to be scolded and told they're doing wrong. If you're going to approach someone to "correct them", there are a few rules you need to follow.
1) Understand ahead of time that, in all likelihood, they'll just "yes" you to death while you're talking and revert back to their own method as soon as you turn around. If they really wanted help, they would've hired a trainer.
1A) Be sure that the person doesn't already have a trainer. If they do, and you contradict that trainer's advice, not only do you confuse the client, but you piss off a co-worker.
2) Be 115% sure they're doing something "wrong" before you go to correct them. Are you pissed because someone's doing a partial ROM seated row? It could be on purpose. Maybe they're working around an injury. Shallow squats? Maybe they're being quad-focused or nursing an ankle injury. Point being, unless you know their entire training program without a doubt, you're only noticing a snapshot. Jumping to a conclusion could be a mistake.
The other day I had my girlfriend doing overhead cable reverse shrugs (think pulldowns without bending your arms, for scapular retraction/depression.) Now, if you (or an uninformed person) saw her doing those from across the room, you'd think she was just some spaz with horrendous pulldown technique. Truth is, she's addressing some shoulder issues. So, what you see, might not be what is.
3) If you've decided to approach them, phrase your comments as being more helpful, not just, "You're wrong, I know what's right." Instead, try, "Hey, sorry to interrupt. I was wondering if I could show you a variation of that exercise that works LMNOP muscle more, and is a little safer for your XYZ."
That approach is more likely to get you an in, but still, don't be surprised if they respond with a "No thanks, this works fine for me." You can't save 'em all, man.
Training friends is a little different. Either flat-out stop helping them if they're just going to waste your time or take the time to explain why deep squatting is better (if you believe it is) and, in this case, explain how it'll help his hockey game.
Can you imagine if a math teacher said "Johnny, you need to study your multiplication tables." Johnny says, "No, Addition and subtraction is all I need to learn." Same principle applies here. You're a teacher, clients are students. Teach, instruct, and hope that some of it actually sinks in.
Few. Few people want unsolicited advice. This shouldn't be a surprise.
For what it's worth, when I used to work in a gym, wearing the shirt with "TRAINER" on the back, I learned quickly enough that the benefit:hassle ratio of giving out unsolicited advice wasn't worth it at all. I saved my advice for clients, that way, it's more valuable.
When I'm doing my own workouts, I don't bother addressing other people's training unless they're in immediate risk of injury (which rarely happens, surprisingly/fortunately). If someone's benching and struggling to get a rep up, I'll pay attention and get ready to bolt over if it comes down.
But even when I see round-back lifting, I don't leap to the rescue, because it isn't necessarily a dangerous thing (depends a lot on the individual, the exercise, and a few other factors generally not worth stressing over at the time.)