Something to consider when offering advice about anything physical...
Not everyone is built the same way, and people can be sensitive about it if it seems like you're pushing a one-size-fits all solution on them. Advice meant for a statistically average person can be dangerous to people with unusual builds, and stress all the wrong muscle groups. For instance, someone who is very barrel-chested may wind up benching with their neck muscles if they use an average-width bench grip (yikes!). As for stomachs, some men have big bellies even when they have a six-pack, so you may have misinterpreted what was going on. (I knew a guy who was built like this, and he hated going shirtless at the beach even when he was the fittest person there, because he was embarrassed by his belly. I don't think he would have liked having someone at the gym point it out.)
If you feel you must give advice about lifts, focus on which muscle groups, etc, should be feeling stress, and which shouldn't. Or perhaps, "Doesn't it hurt your ___, doing it like that?" You can express your concern in a neutral, pleasant way. If your guy's 'awkwardness' was due to an unusual build, you probably came across as a self-appointed expert on someone else's body.