my friend, being a giant white knight, tried to help her
I can be completely wrong about this, but it sounds like he was helping her because she was his girlfriend and wanted her to like him more. He wanted to look like the "giant white knight." I think that's the problem, instead of genuinely wanting to help just to be a good friend, he wanted to up his rep.
What usually helps solve these issues is to apologize in case you did something wrong (and to not do so with a sarcastic tone- even if you are completely innocent) and try to make things right. A lot of the time if I apologize to someone angry at me for no good reason, it usually helps them to calm down, and then they see that I wasn't the one doing anything wrong and they later apologize to me as well.
But also- this is why I'm waiting until college to have any sort of serious relationship. Most of the time High school (and younger) relationships just end up in drama, hurt feelings, and wasted time.