Riding over to a mate's house one evening, I caught up with a guy on a nice, but badly set up road bike who was making pretty heavy going of it. Given that I was just wearing casual shorts and only going about 3 miles, I didn't say anything. For all I knew he could be just finishing a double century or something. Anyway, we got chatting and he confessed that his knees were killing him, he was just trying to get home.
Of course, that gave me the opener to suggest that his saddle looked a little low and if he raised it a bit, his knees might not hurt so much. He was very grateful, asked a few questions about saddle height etc and said thanks.
But without that opening opportunity, I'd have kept my mouth shut. Nothing worse than someone coming along and offering unsolicited know-it-all advice.