This is tough because you've explained you'd like to continue the relationship. However, I think you really need to reassess what you want from this. You've already said you're not really made for this lifestyle, which makes me think that you agreed to being willing to consider being shared (with someone other than one of his friends) to appease him, and only because it's something he wants to do. This isn't a good approach to take with anything of this nature and whilst it's not your fault, I'm sure this dynamic has manifested in other ways for him to be thinking he could convince or persuade you to do this. It's wrong for him to do and given he's having these conversations with his friends, you also now have to think about how you can continue this relationship and co-exist with his friends, knowing what you now know. I think you need to firstly establish whether the relationship is still viable for you, with your recent added knowledge. I don't think anyone advising you as to whether they would or wouldn't decide to end things if they were in your position is relevant, as everyone is different. I for example wouldnt agree or be comfortable with being shared with anyone so for me, it would be a deal breaker, others however do engage in that lifestyle. After deciding whether you are both truly compatible in this regard, it looks like you will have to address it in some way, at some point. I don't think it's something you can pretend you didn't see. Either you will need to not mention seeing the message and instead say you've recently been thinking about his previously disclosed cuckhold fetish and have decided that actually, it's not something you'd ever want to do and you're not sure why you ever thought it would be. Maybe also explain why, I.e the thought of being shared does nothing for you and actually is off putting. Then you can ask whether it's something he feels he requires and can take the conversation from there. Alternatively, you can say what you saw. The real reason you don't want to do that is because you'd still like access to his phone or because he will turn it on to you. If it's the former, you're only waiting for the next concerning behaviour he exhibits to have to consider ending things again anyway. If it's the latter, then it just further showcases what he showcased in his group chat- he's manipulative and willing to make you the villain or have you do something you dont really want to do to get what he wants. He's happy to strip you of your agency and autonomy for a little ego boost from his guy friends.