Yikes. Your friend making YOU feel bad for not unconditionally cheering on her choice to help someone betray another woman - that's not feminism. She was gaslighting you making you think you need to work on yourself to change your own feelings about her behaviour.

A true friend is honest. And sometimes friends do not agree, sometimes friends say "I love you, but I don't support your choice and I think it's wrong". And if that's a dealbreaker then that's sad but you have no obligation to change your morals and try and brainwash yourself into thinking infidelity is a good thing because being judged makes your friend feel sad.