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All the fucking time.
Seriously, we have far too many examples of this happening, starting from the fellow street kid one PC rescued from a burning building once and since befriended. Other examples include a powerful jungle witch intended as BBEG, before the same PC switched sides and decided to become her disciple instead, at least partly reforming her in the process.
The party has also shown sympathy towards defeated bandits on at least three different occasions, teaching one of them to fight better so they could enter a streetfighting tournament, helping another integrate back into normal society by taking up defense responsibilities in a newly founded frontier city, and trying their best to make sure the third one manages to escape from a doomed city despite him trying to attack and kill them only moments before.
And don't get me started on the joke character trio of Bell, Book and Candle (pic related), the players met them exactly once, and the next thing they decide to do is travel all the way to another continent to liberate them from their evil masters. During the process they fought some other NPCs, including a group of enemy knights as well as a giant berserker, and recruited some of them to their party as well.
Finally, in our last adventure, far beneath the aforementioned doomed city, the party knew there was an adversary far out of their league awaiting them. They could've turned back, but the enemy was holding captive one NPC they had once been on a mission with. They didn't even like her very much, having had some disagreements over chain-of-command issues, but fuck if they were going to leave her there without doing nothing. The street kid I mentioned in the earliest example fell during the rescue - they took it pretty hard but still considered it a worthy death.
So basically, in 4chan parlance my group would be a huge bunch of moralfags, but it seems to be working out well for them.