Some do, some don't, I might
My old commute, I'd usually pass a couple of cyclists who I got to recognise. Most wouldn't make eye contact with the smiling lady. One guy, however, did also smile, and then we'd wave, and then we'd shout 'MORNING!' across the road at one another. He's a guy who is always out running or cycling in the area.
New commute, on the way in I've yet to see another cyclist. However, I do pass the same gentleman out walking his dog. So we say Good mornnig, even though I'm going up a hill and can barely breathe. On the way home, about half the time I don't see any cyclists, and about half the time I do. As before, if I do see them, then I'll smile. Usually it's not acknowledged, but the other day a Proper Roadie smiled and nodded at me in return, which absolutely made my day (but then again, I probably look like a bit of a roadie myself. Until you seem me on road that isn't flat or downhill).
As for stopping if another cyclist was in difficulty - I imagine that I would without hesitation stop. Which is daft really, because I'd be of no mechanical help whatsoever (but, you know, I'd have my multitool and spare tubes which might help if they knew what they were doing but were without equipment).
Sitting here thinking about it, I'd probably wonder if, as a lone female, I should stop, but if it actually happened that thought probably wouldn't enter my head (like when I was at Paddington rail station, sitting waiting for a train, the bloke next to me asked me to keep an eye on his stuff while he took himself and his dog off to buy a newspaper. Sure, said I, given that I was sitting there for the next 15 mins and he was going to be about 2 mins. Once he was gone, of course, then I wondered if I was sitting next to a guitar-case-shaped bomb...)