If it's a complete stranger, I'm unlikely to offer any advice. Even if they're doing something I think is dangerous I'll still generally keep quiet unless it's an easy fix.
If it's someone who I know in a non-cycling related capacity, I won't offer any unsolicited advice but will happily bore them to tears if they bring up the topic of bikes. Or anything vaguely related to bikes, roads, round objects, breathing etc.
If it's someone I don't really know on a group ride with me, I'll generally keep quiet. However, I'll speak up if they repeatedly make safety-impacting pack etiquette mistakes. Hold your line, stop coasting etc. I won't talk about riding style or training or style or whatever.
If we've been on a few rides together, had a few beers together etc it's open season. Raise your saddle. Lube your chain, the squeaking is driving me crazy. Stop grinding a 53x11 at 19 mph. For the love of God, turn that barrel adjuster. Bring your own tube next time. Stop ducking your pull, dead wood. I expect to receive the same.