If it's a guest posting, it's a real headache actually - I had to download the entire log - 22MB
which is one hell of a big file. I tried saving it, but ran out of RAM at the wrong moment and had to reboot and start all over again! Downloading took nearly 15 minutes each time I think. Even if I had saved it, I don't think I could have opened it to read it anyway, as it was so large.
In the end, while it was still in the browser (in the Windows temp folder) I searched for the word "Replied" (using CTRL+F), starting from the end of the file and working backwards. Fortunately, as not many people had posted after the incident, it wasn't long before I came across a series of guest postings, all with the same IP#. That identified our friend with complete certainty.
I could not have searched for this, using the normal search function in the Admin Center, because I didn't know the IP# at that time and there was nothing else useful to search on.