Yeah Childhood Friend, you reap what you sow. First talking him down, saying he doesn't look good, girls don't like his appearance, flirting with his gym rival in front of him, giving him the cold shoulder sometimes... All of that even though he obviously had a thing for her, but as soon as he turns to other women, she get's jealous.
MC should bang blonde girl. And if FMC/childhood friend (she might have already done it with the douche/gym rival) now does it with someone else out of jealousy it doesn't matter anymore, since he did too but at least he enjoyed it.
The story would have been a little bit better if MC and childhood friend didn't do it together. When she was defensive like not wanting to do it that night he should've just stopped. Would be a little bit better, then she wouldn't have even the least right to be jealous now because she was clearly denying him that night.