The general rule, which seems to jive for most golfers, is don't give out advice unless asked for it.  I've seen this situation happen and also been involved in it.  A foursome is mid-way through the round when one guy suggests to another guy, (who he just met at the 1st hole) he's too far away from the ball and the reason he's shanking shots.

I'm going to be as blunt as I can about this situation.  Unless you're a teaching professional, a tour pro active or retired, golf coach or scratch golfer, don't offer unsolicited advice.  It's one thing if the struggling golfer asks you specifically to take a look for problems and asks for your magic cure. 

I'm not sure how many times I've seen a 10-20 handicapper offer advice to someone who didn't ask for it.  More often than not, the struggling golfer gets pissed off.  Simply because someone of similar skill doesn't get the same street cred as the experts I named above.  Even if you're one of the experts, I've played with enough of them to know they don't like to give out advice anyway.

If you are a golfer who wants to give advice and fit the golfer I'm describing, you have two options.  One is keep quite and enjoy your own game.  Two is wait until the struggling golfer gets so frustrated they ask you to check out what they're doing.  Maybe you can spot something to help.

Please do your part in helping the golf community rid itself of this annoying character lurking about our fairways.