Not an easy solution I'm afraid. You can only really do this by re-searching for files you know the other host is sharing.
It is possible you have actually blocked the other host, by clicking "block host" instead of "send" ... I've done it myself
in which case their ip will be in your block list, but it depends on how many hosts you have blocked, if you can find them, and you will still not have their port number (the number after the : colon
If you start to chat with someone via a search result or download/upload result, it is always a good idea to make a note of their ip number, although this is not even necessarily a useful thing to do as many peoples' ips change regularly. and of course this is not much use to right now....