Who says we can't be friends?

Just because he was in Slytherin and she was a Muggleborn didn't mean they couldn't be friends: that's what Regulus kept telling himself anyway. And Lily already had a friend in Slytheirn didn't she, so it couldn't be that hard? However, as they get older, things get more complicated. Can they still be friends on opposite sides of the war? 1972-1979.