Seeing a fellow rider stranded by the roadside is always an opportunity for spreading fellowship and altruism.
If spreading fellowship isn’t your thing, you can always make sure you shout, “You alright, mate?” late enough in the ride-past that you won’t be able to hear the answer.
Certainly that’s what I do.
If you do stop, remember the oath of the amateur mechanic. No, it’s not “f***ed”, it’s, “If you can’t fix it, at least try not to make it worse.”
Many is the stranded rider who started off with a broken spoke, but who found a passing angel to add a bent rim.
Resist the temptation to provide a free product review service. No one wants to hear, “Ah, you see what you’ve got here is the XL model, which tends to do this on rough roads. If you’d shelled out for the XLF this would never have happened.”
In a similar vein, avoid critiquing your castaway’s workmanship and/or personality.
“Of course, if you were a proper cyclist you’d have set the limit screws properly and neither of us would be standing here getting mud in our cleats,” is not in the altruistic spirit of the ministering angel.
Do not attempt a repair you’ve only ever seen a video of. I know it’s not your bike, but still, you wouldn’t go and practise your w brain surgery on someone else’s child, would you?
Be prepared to lend someone an inner tube and accept that you’ll never see it again. This is how the world works.
(Except in the US, where they recently voted against lending each other inner tubes.)