Different people learn differently. For me, it often took making the mistake on my own and suffering the consequences to learn. Flying with an instructor/captain/more experienced pilot who always pimps you on little things is great, and makes the flight seem easy, but for guys like me, it doesn't make for quick learning. I'd say hang on as long as you can possibly take it, and try to see if the guy figures it out, or at least bails himself out of the whole he has dug. Then debrief it professionally post-flight. I'm sure there are some special considerations when carrying paying pax or if on a revenue flight, but if we're just talking about little headwork/cockpit management/SA issues, I'd say the guy can probably learn better by making his own mistakes. If safety of flight becomes an issue, that is a whole other bag of worms of course....