I need some advice on this situation because it is stressing me out the closer it gets to their wedding, please advise (also this is my first post so I apologize if idk wtf I'm doing and apologize for any mistakes). So I have a tight-knit friend group from high school that has stayed in contact since we graduated 6 years ago and hangs out 1-2 times a week. This friend group is co-ed with many couples in it, and we all enjoy going on hikes, going to the bar on the weekends, and typically have next to no drama within the group.

When we all split off to go to college, a new couple was added to the friend group: Will (26 m) and Amber (26 f). Will joined the friend group first and was on and off with Amber at the time he joined the group. They were off and on constantly because he had been doing some shady stuff/ cheating in the past already (dancing with women at the bar, turning off his location on a night out, taking pics at a bar then going next door to the dance club and lying about where he was, etc.). Anyway, Will apologized to Amber for cheating after we all graduated college and to win her back he moved to be with her in the city she moved to. I do not know the extent of why they broke up before this and have never been super close with either one of them to ask.

Now, the two of them make regular appearances at hangouts and live with 2 dogs in an apartment together in the city. Amber is one of the kindest people I have ever met and works as a NICU nurse- she is literally an angel and one of those people that just make you smile talking to them. Because she works as a NICU nurse she typically works the night shift during the week, which makes it difficult for her to go with us when our group goes to the bars on the weekend in particular. I myself don't go to these bar outings more than once a month and hardly, if ever, have seen Amber at the bar with all of us. This is where the problem comes in.

Will cannot control himself when it comes to other women, especially while drinking. He casually buys women drinks and closely talks with them at least once every time the group goes to the bar without Amber. This isn't just a nice "oh he was having good conversation with a person at the bar who just so happens to be a girl" it's "why are you buying this girl 4 drinks and touching foreheads with her, especially in front of your friends" There are many instances of this happening where he does something an engaged man shouldn't really be doing, and I hear about it through the people that went to the bar with him afterwards in a gossip session. It's kinda like "shocker, listen to what Will was doing with X girl at the bar."

A couple of months ago, Will proposed to Amber, and she said yes. Will never told our friend group he was going to propose and was hanging out with us the night before he popped the question (which I think is very odd). We all hoped that since he proposed, he would stop flirting with other women and making us all uncomfortable, but that has not been the case, and it is causing tensions to rise.

Will's bachelor party was this past weekend and he invited all the guys from our friend group to join and go fishing. Apparently, while on the trip, he disappeared while they were all out at the bar celebrating, turned his location off, and was found closely talking to a group of women and buying them drinks. I am at my wits end and I don't want to hide this from Amber anymore. It makes me feel awful knowing she trusts our friend group and has even said to one of the other girls before "I am so glad when Will goes to the bar with you guys, because I know you all will keep him in check." They are getting married in 3 months and I want to tell her but our group has been arguing on how best to tell her, if at all.

Half of the group thinks that the couple guys closest with him need to sit him down and have a serious talk with him (one girl in our group did this in October but no improvement) to take his engagement seriously and the other half think we should have a girls only night with Amber and tell her before they get married. The two of them have already secured a house together, the wedding is thoroughly prepped, and he has done this in the past and went back to him. I am scared of telling her and she turns on us because while I think she is awesome, Amber and I aren't super close. Is it my place to tell her, even though we aren't close? If we tell her in a group setting will she feel ganged up on? Do we tell Will if he slips up again, we are telling her? I don't know the right answer here and would love some advice on what would be the right thing to do. Am I the asshole for doing nothing?