TLDR; friend was used to hide cheating, I point it out, I get called an asshole for the insinuation that her good nature was taken advantage of.

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My ex invites out a "friend" that I didn't know about without any mention of him to me (I didn't even know they were friends tbh) to a hike with a mutual female friend of ours. I was not aware that they were going out for a hike at all. 

Yes, she had gone out with other male friends before, I never said anything because she was upfront about who she was going with and when and I trusted her.

They all go out, whatever, I find out about this hike, two weeks later, gf and I break up. A week later she's dating this "friend" 

First of all, that's cheating. You're hiding the fact that you invited someone else out, and purposefully excluding me from this outing. Not only are you hiding it by not telling me, you're bringing along a mutual friend of ours to cover for your fucking cheating in that "they were just friends"

This friend of ours is adamant that my ex didn't cheat as "emotional cheating doesn't exist" and I feel she should stick up for me to the asshole that is my ex. Not only is she more or less saying that I'm wrong (because not cheating) but I'm an asshole for pointing out that she was taken advantage of by being a cover to hide it. 

Hard for me to see any other way as no word of this friend was ever spoken to me, and I wasn't even aware that they were going on a hike. 

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AITA? I feel bad that she was put in the middle of this shitstorm and I'm even more pissed that my ex straight up used her.