Ah, yes, the quandary and risk of offering constructive advice. (No good deed goes unpunished.) Being female makes this even more fraught.
But I’ve found that approaching someone with really bad form can sometimes work when you use an empathetic approach. I’ll usually start by saying I’ve noticed they seem to be really struggling and that I worry that their technique may cause them injury or hurt. I then go on to tell them that advice that I got from a more experienced paddler (or instructor or guide) had really helped me paddle with less effort and more efficiency and suggest maybe they would like to try some of their tips and see if they work for them,
Unfortunately, too many people are embarrassed and even freaked out about feeling like they are being “judged” or “corrected”. That’s usually what prompts stubborn pushback and deflection. Being “corrected” by a woman can bruise a man’s ego and being “corrected” by a man can trigger some women into feeling they are being “mansplained”.
What I try to convey is “I used to do that the same way you do but somebody with more skill taught me some tricks that made it easier. I’d like to share those with you to see if they could help you out too, if you’re OK with that.” I’m very careful to never tell anyone they are doing something “wrong” or “incorrectly”. Even with the ubiquitous newbie-with-the-upside-down-paddle instance – my ploy with that is “hey, here’s a quick tip – try flipping that paddle over and using the “scoop” side: much faster.”