My (25F) boyfriend (25M) groped my best friend (24F) after a big night of partying. What do I do?

TW, obviously.
Pretty much as the title says, but I'll provide more details. Its long so be prepared.

There was a party on the weekend that myself, my boyfriend ("Mason"), my best friend of 15+ years ("Trish") all attended. We all previously lived in a sharehouse together as well for approx 2 years about three years ago.

I arrived late to the party due to a different event and left early because I had work the next day. It was clear during my time at the party that Mason was inebriated. He had asked me before the party if it was cool for him to do some molly with some of our mutual friends, and I said sure. He was clearly under the influence of this when I was at the party, in addition to a heap of alcohol (it was a party specifically oriented to drinking games), but he could talk with me and hold a conversation. When I left, he declined coming with me, but we both said we'd see each other in the morning. I went home, slept, and got up for work. He was home at this time (approx. 6am), just coming to bed.

On my way to work I received a call from Trish, who sounded super upset and distraught. She told me she didn’t want to jeopardise our friendship but something had happened and she needed to tell me. I told her nothing could do that – we’ve been friends since first grade, and through a lot of shit. She proceeded to tell me that after I left the party, Mason had become touchy-feely with her – touching her hair and face. This made her uncomfortable. With the party antics, Trish ended up doing all of the boys’ makeup, including Mason’s. During this time, Mason was touching her leg. Trish redirected Mason a few times, but eventually just stopped doing his makeup due to feeling uncomfortable. Around this time, the party was wrapping up, and there were talks about getting rides home. Trish offered to drop Mason and another friend, (“Paul”) home. Mason sat in the front seat, and during this time he was touching Trish’s leg again. When Trish pulled up at our place to drop Mason home, he ended up swapping seats with Paul, and decided instead he wanted to go along to Paul’s house. 

Sitting in the passenger seat behind Trish, Mason began to touch Trish’s head, then side of her face, then shoulder, then neck/chest area. Trish said she froze up. He then slid his hand between her shirt and bra, and touched her breast. Trish slammed the brakes, which pulled Mason away. He didn’t do anything until they got to Paul’s house when Paul’s girlfriend told him to leave, noticing that Trish was upset. He came home after this. Paul followed to talk to him about what had happened.

I’ve been reeling and numb from this news for the past two days. I can’t fathom it. I’ve been with Mason for 6 years. During these 6 years, nothing has ever made me question his loyalty. He doesn’t even follow models on Instagram. He is usually, as a rule, super respectful of boundaries and is defensive of me and my girlfriends when we hit clubs together – he has approached and cussed out other men who have violated the boundaries of my friends multiple times. He does however, have a habit of overdoing it when it comes to drinking and indulging in substances – something we have talk about before from a ‘that’s not good for your body’ perspective only, and something that has been brushed away by him a bit. I 100% believe Trish. She wouldn’t lie, and she was so upset over the phone. But I am having a hard time piecing this all together. Why would he do this? Through our relationship, his subsequent friendship with Trish, and our stint of time living together, there have never been any issues or suspicions. 

I confronted him when I got home from work.  He said he has no recollection of anything, other than Trish slamming the breaks and snapping him to attention. He recalls Paul’s girlfriend telling him to leave their house, and asking why, hearing Trish crying, then agreeing to leave. He says he doesn’t know why he would do something like this, that he has always seen Trish as only a friend. He explained that he took another molly and played another few rounds of drinking games after I left, and that’s when his memory dropped off. In retelling him what Trish had told me, he did not recall getting his makeup done. He is remorseful and was teary all through the retelling.

I understand that molly can make you excitable and touchy-feely, and I understand alcohol impairs judgment. But can I stay with him knowing he did something so devastating without consent? And to my best friend? How do I know that this isn’t part of a deeper-seeded issue, that maybe he’s not aware of boundaries like I thought he was? I feel sick. My values as a woman tell me ‘cut him off’ and that’s certainly what I would tell another friend in this situation. But it's six years of my life with him. My love for him hasn’t just evaporated since hearing this news. I really thought he was my person, and that we would be married in the next year or so. Part of me wants to give him a chance and see how he can grow from this, if he can make things better. I do believe people are flawed and make mistakes. But how can I do this without severely hurting my friend? Is it better for me to just break up with him? 

Trish has said it will take some time for her to feel comfortable being around him again, and it certainly won’t be when alcohol/substances are involved. Mason has told me he will never take substances again, and has recognised that his drinking is problematic – not just the volume he consumes at parties, but what he drinks through the week too. He has messaged Trish and asked to meet up (if she’s comfortable) and chat things out. She has declined at the moment and he has accepted this.

I don’t know what I want out of this. Feedback? Advice? Have you experienced something similar? I feel so alone in this and don’t want to talk about it with anyone, because I feel like it's going to be interpreted in a biased way by people who are friends with all of us…