@traumatizedntrying,
It wasn't.
She likely has an idea that he's a jerk although she may not know the specifics. Or maybe she knows all the specifics and still chooses to be with him. That's her issue. It's not yours. Either way, she didn't need to hear from you in order to make relationship decisions for herself.
Usually, when someone gets this kind of message from a bf/gf/husband/wife's ex, the first thought is that the sender has an axe to grind. And you do/did. But you weren't doing this to help her out in a show of sisterhood. You were doing this to try to feel better about
yourself.
Don't do that.
I sincerely hope you are getting therapy. You're not in a position to just heal yourself magically, at least not from what I am seeing (IANAD).
You're giving him rent-free space in your head. You're worried about what he'll say, etc. You're even worried about his friends judging you, and they are how many hundreds if not thousands of miles away?
That's like the mayor of a small town on the other side of the world being worried that you aren't voting for them.
I strongly encourage you to work with a therapist. And if you are already, that's great! But give it more time.
And if this guy comes around looking for another booty call in the future, tell him where to step off.