tell them what they can do better.
Easier said than done. Most of them won't even understand english. That's another reason why i try to avoid open games: first make sure the person is adequate and speakable to, then play with him.
no one likes being called a noob even when they know its true.
Unfortunately, quite a lot of people enjoy it. "Ah i'm not some idiot who spends his whole life practicing the useless game, I just play 'for fun' and it's ok for me to spend all my money on building bunkers in the middle of my base since i find it 'fun'", they say.
and vice versa.
No. Low oil theory includes all stuff required to play high oil; a good low oil player has no trouble adapting himself to high oil, it takes only two or three matches to adapt, at most. On the other hand, people who only enjoy the limited theory of high oil games are usually dying in under 6 minutes in any typical low oil 1x1, simply because they have never thought about things they need to think about there; an huge re-learning is required for them to play well.