AITAH for asking my friend to apologize for “ruining” a birthday party?

For privacy reasons, I’ll be using code names for the people mentioned.

So a while back, my friend \[F, 18\] was hosting her 18th birthday party at her grandparents cabin in the woods near their house. She invited a huge group, consisting of me \[F, 18\],my two other friends \[F, 18 and F, 17\], and two of her friends \[F, 18\] that we were familiar with. During the evening when everyone had arrived, the birthday girl had a bunch of games that we could all play, ranging from board games to various group activities, however one of our friends (I’ll call her Pepper) wasn’t coming out of the room to play or even hang out. We had extended invites for her to come in and chill with us, but she would usually decline. Even if she declined, we still let her know that she’d be able to join us if she wanted too (which she did, a couple times) even then, we always tried to make sure she was included, especially the birthday girl herself.

However, the problems started to happen the morning after. A lot of us were in the living room cleaning up, eating breakfast, and packing up our things to leave. One of our friends and the birthday girl went to go check on Pepper, assuming she was still asleep. A few minutes later, they both came back with the birthday girl crying and our other friend looking unhappy. I asked what happened and the birthday girl explained that Pepper was in the room refusing to come and hang out with us before we left, and that she was also crying. I soon went into the room and talked to Pepper, wanting to figure out what was happening. Pepper went on to explain that she felt “unwelcomed” and nobody liked her, because no one wanted to hang out with her in the room. I told her that we do want to hang out, but we don’t have to do that in the room when we can all hang out in the living room together. She said no, and insisted we had to hang out in the room. I ended up walking out, too frustrated to keep talking and afraid I’d make the situation worse. Before I left however, I asked her to extend an apology to the birthday girl, because she had really upset her. I also believed if they both talked it out, things would be fixed.

Pepper came out of the room a few minutes later, a little better than before, and she went into the kitchen where the birthday girl was. I didn’t hear the conversation, but when the birthday girl came back, she told me that Pepper didn’t apologize to her, and instead told her it wasn’t her fault for being upset because ”her friends didn’t make her feel welcomed”

I really don’t know what to do. I’ve tried talking to Pepper about it, but she has dodged every attempt I’ve made. This isn’t the first time she’s caused problems, but that’s a story for another time. So, AITAH?