How to view chats from unfriended users?

Ok so I was talking to this one guy that I was friends with for a long time. We got into an argument and I unfriended him and removed our chat by clicking the "x" button next to it. I later regretted it and wanted to talk to him again but I don't know how to open chat messages from users that I unfriended. I was waiting for him to send a friend request but he didn't send one. So I check the servers I was in for his username but he wasn't in any of them. I know his username, I just don't know the numbers after it. It there a way for me to open chat messages from people that I have unfriended? Maybe a feature, api or plugin I can use?