Hi Jenny01, whilst it can be a lot and frustrating to hear what your friend is saying right now, it is important to be mindful that while you can see things at an arms length and the logic of it all, right now it sounds like your friend is hurting a lot as this is a very new and raw emotional hurt. This is a really common emotional experience of the end of relationship, particular if you aren't the person who wanted it to happen. It is a kind of loss, and as with other kinds of loss, we need to be able to grieve an process, does that make sense?
Heather, the founder of Scarleteen, wrote a fantastic article about handling break ups here: Getting Through a Breakup Without Actually Breaking
, whilst it is addressed to people going through break ups, this could help you understand the emotional experience he is going through, and it might be a useful thing to show him.
Perhaps, right now, rather than explain the logic to him to try and make him stop expressing his emotions, the best thing that you can do as a friend is listen, and validate his emotions, as much as feels comfortable for you. When people vent like this, often they are seeking validation and listening, rather than advice or answers. If you feel you can listen in this way, it is important to look after yourself as well, and make time to emotionally recharge as it can take a lot of mental energy. If you need some time to not talk about this with him, if he is getting repetitive, you could redirect and suggest an activity that could help him with distraction or something you both enjoy to take his mind off things.
It is okay to have a limit here and say something like: "I'm not in the best headspace to talk about this right now, but I can definitely do/talk about [something you both like] because I'd like to help you feel better as your friend". As a friend, one of the important things about how you deliver this is with kindness, so that he knows you aren't rejecting him 'as well' (as any breaking up, even if totally logical, can feel like a rejection) and that his feelings are valid, just that you aren't best placed to help him vent, but still there to support him as a friend.
Does this make sense? Let us know what you think about the article and if we can expand on or clarify anything.