I happened to walk by a man and woman casually playing informal unscored tennis, somewhat above beginner level, at a local park. It was immediately obvious the woman was waiting far too long before beginning her strokes, basically letting the ball almost reach her, even at the baseline, then attempting a quick, cramped stroke without much power or control. The couple was also doing something I see far too often at the park: frequently calling "out" and ending play instead of playing the ball anyway to try and keep the "game" going, keep up the rhythm, and getting in as much practice as possible instead of constantly stopping/restarting.
I was tempted to introduce myself, ask them if they wanted some unsolicited advice, and if so suggest the woman try to start her swing as soon as she sees what side of her body the ball will be arriving at (although she'd probably need practice changing her style but might end up with much better strokes), and also to simply try to continue playing when calling outs.
My wife said I should mind my own business, because I have no idea what sort of relationship, advice, practice, and coaching they've already gone through to reach a level of play they might be perfectly happy with. On the other hand, my personal experience, for life in general not just sports, is that I've rarely received unsolicited advice at all let alone bad advice, and what unsolicited advice I have managed to receive is usually very helpful. So from a "golden rule" perspective I would tend to want to give advice that I myself would want to receive.
What's the proper etiquette? Is there some best or proper way to offer playing advice to strangers? Or should I keep my mouth shut?