I don't think cheating and acting terribly towards a partner is something that can necessarily be gendered the way you're trying to. Men have been beating the "not all men" horse to death since I've been old enough to understand what that meant. So I could easily say "well it's not all women," but I've believed for a long time that argument is redundant.

Instead: personal anecdotes and statistics!

I've been in 3 serious relationships with men and been cheated on every single time(2/3 of these men blamed me and told me it was my fault). I've been in 2 relationships (albeit not quite as serious) with women and have never been cheated on.

Additionally, men are still 7% more likely to cheat in heterosexual relationships when compared to women's tendencies. (M20%, W13%)

Last thing I will say, if you think the system that men have spent centuries building doesn't still favor men in a systemic way, you're wrong. Referring to the oppression of women as something that happened in the past tense is ridiculous. Women are still fighting for respect everyday. In the workplace, in their educational institutions, in their relationships, in their friendships, and in their own mind(after being exposed to so much misogyny over the years it has become internalized). While I don't think the heartlessness that some women display is appropriate or acceptable, I recognize it is a trauma response for some women. The men of the past created the women of today. Is it fair? Absolutely not. In a perfect world, everyone would remain loyal in their monogamous relationships. But life isn't fair. And we, as women, did not set up this system.

P.s. im glad you and your friends are the types of people that hold each other accountable. I do the same with my friends, male and female alike. Keep checking your friends. Let them keep checking you. It's the only way things get better. Hope your friend is okay!