Why does he acknowledge me and not her even though he got with her?

so this guy who i had a thing with was trying to get back with me...then he ended up getting with one of my best friends. i found out about it and was pretty disappointed in both of them but i settled things with her. he found out that i knew about it, but when me and my friend( who got with him) pass him in the hall, he completely ignores her and just says hi to me. i don't understand because like wouldn't he feel bad that he did that and not even bother trying to be friendly with me? and like i think he just wants ti be friends now