I think we already covered that... she was just nice, there doesn't need to be anything bigger than she did a nice gesture due to being a nice person.
She saw a little kid get bullied by a stupid asshole, on his birthday, she sees this kid sad and dejected... she tries to cheer him up...
I don't feel this warrants any further explanation, she saw a kid feeling bad(someone she knows... at least a friend of a friend) and decided to cheer him up, sounds like something any humane person would do... had she simply walked past him ignoring his suffering it would be the actions of a self centered cunt.
As for the next point i can only speculate, maybe Erynn told her this was his birthday before the flashback even starts "i have to stop by MCs house later, it's his birthday", maybe the same thing a few days earlier... but there isn't a single solid thing telling us how she learned this info.