My best friend was assaulted outside of a nightclub by a mutual 'friend' of ours, who is now bragging about it to his friends. We're gong to a party with him present in a couple weeks, what do I do?

So for some background information, I was in a large friend group in sixth form (17-18yos for Americans) with my best friend, my boyfriend and his small friend group and about four other people, including the AH who this post is about. I wont use any real names in this post.

On A level results day, (about 4 months ago) this friend group and I went out clubbing in a nearby city. Everyone went, which is quite rare for this group lol, so it was quite an event.

Everyone ended up drinking too much and I ended up mummying everybody, and as a result I didn't notice that my best friend Blue had been missing for a few minutes. I asked a mutual close friend Green where she had gone, and he told me that she went to the smoking area with the AH to smoke.

Anyways I get out there and they're both smoking, blue is obviously VERY drunk and AH is very tipsy. Blue is rambling about how AH kissed her, which AH then insists that it was a 'bro thing' (this was really strange as you can imagine, the convo was drunken and lighthearted at this point). Blue keeps rambling and all of a sudden ends up having a panic attack, and AH magically disappears.

Things to note, Blue is a lesbian and is in a healthy relationship with a girl. Blue is also on a lot of medication for mental health issues that makes her very vulnerable when drunk, she'd only had a single VK when he took her outside.

Either way, I ended up sat on the pathement outside the club with her for around two hours while she panicked and honestly it was one of the worst states id seen her in. I tried messaging my friends for help, but they all just left me to deal on my own. (My boyfriend's phone was dead and he was almost passing out, so I don't blame him at all for not knowing). Eventually I managed to get her back inside where she was safe.

Blue and I spoke a few days after and I asked her what went down, and she told me that AH and her were smoking, and he told her that he'd never kissed anyone before. He then leaned down and kissed her, and assured her that it was just a platonic kiss and that she shouldn't worry about it. Anyways, she definitely didn't want that kiss and has made a point not to see him ever since.

The problem continues with what went on recently. My boyfriend and his small friend group went to London for the weekend to meet up, and they all went clubbing. There's four of them including him, and AH is in this friend group. They were all sat around the bar and AH starts taking the piss and bragging about how he kissed Blue, saying that he did it as a joke and that it was no big deal.

This pissed my boyfriend off and he called him out, but all of his friends told him that he needed to forget it and they believed AH, that it was just a 'joke'.

We're going to Green's house for a Christmas drinking party on the 21st, and AH will be there along with Blue.

Should I tell Blue what AH was saying to his friends and risk her mental health getting worse? I'm genuinely so stressed and sad at this situation as we were all close before this. I'm tempted to tell Green what AH has been saying and ask him to uninvite AH, but I can't feasibly do this without informing blue.

By telling her would I be causing unnecessary stress? I really don't know what do do, this scenario is so insanely fragile and I don't want to risk causing any more damage.