I too learnt a very valuable lesson about life a few years ago. I was fishing the Trent in a club match at East Stoke, and was having one of those special days you get every now and again, with a nice roach almost every chuck. The bloke in the next peg downstream of me, hardly had a fish, so I decided to go try and help him out. BIG MISTAKE! He told me in no uncertain terms to chuff off and mind my own business. Lesson learnt, although I meant well, what right did I have interfering with his day? I've never forgot that, so if I think I can help someone to make their day better, I always ask them if they mind first.
The bailiff on one of our club stretches has asked me to go and speak to struggling anglers a couple times recently and I have thought that whilst it would probably be fine there is a chance of a situation like the one you describe if probably not quite so blunt. As its happened they’ve left both times by the time I’ve finished fishing which suppose figures if I’ve done well and they haven’t. Still I would have offered to help if they had still been there as it isn't nice to see people struggle but like you say you need to consider how to put it across.
Ill speak to anyone within reason. I also listen to people and its paid dividends in terms of catching. Years ago used to fish a venue near Notts that was always decent but the guy we saw regularly always did well and we couldn't tell what he was up to / using. Over time it became apparent he was using a bait that was smashing the place. though on the face of it he wasnt a trendy match / pole type. he told us the bait and we hardly believed him. though it was clear he was doing something different to everyone else . The bait he told us to use ( we grow to know him ) was ' manky cheap prawns' ... we almost laughed......then bought a bulk bag and froze / re-froze them for a few sessions.....the older they got the better they were. Carp / Tench / Roach / Huge perch took them ..... weve never dismissed a tip to try since