You are right about this, but there are two points to clarify.
1) I would never try it with my friend's girlfriend either, but Erynn is not our girlfriend, she is a childhood crush, adolescent crush. If a friend of mine and I at 13 have a crush on the same girl, but neither of us turns out 5 years later, why should I still consider her off limits? Why do I (Alexis) have to sacrifice myself? Can't Erynn be taking a step back?
2) On the other hand, I am seeing it from the point of view of the MC, and not from that of Erynn. Surely it is right that Erynn is angry with Alexis, in her place I would be very angry too, certainly I would also be angry with myself, since I wasted time and gave the other person a way to come forward. But I'd probably struggle to consider Alexis a friend again, but not for dating MC, but for keeping him from me, I'd say after the gig (When it's clear the relationship is getting serious, or whatever something more than just a flirt) he should have called her and told her the truth. But these are things that concern the relationship between Erynn and Alexis, to me as MC, it can annoy me to feel teased etc, but then with a clear mind I would be able to contextualize everything and get over it. Assuming I developed a serious interest in Alexis (Yes I did)
3 optional) Alexis is the loser who made it, that's why I like her, she's the classic runner-up, condemned to see the guy who loves drooling behind the mean girl on duty, who for a reason or a nothing else appreciates or does not realize it. But at one point he said: fuck the mean girl, fuck everyone, I want what I want and for once the others will drool. Does this make her bad? For me no.
We are always in a fictional world, a video game, you have to maintain some mental elasticity when we approach these games, if I behaved in the game, as I do in real life, I would not see half of the porn scenes (Stepsisters, real sisters, blackmail, mothers , adoptive mothers).