chat room account hacked?

I have a quick question for the tech savvy. My Brother has noticed changes to his account in an online chat room that he is a member of. He seems to think that his ex (recently broken up) may have obtained his username and password either by snooping around or by accessing his computer remotely using some kind of keylogger or similar. Can he find out who has accessed this forum and if he can, can he press charges? He is really pissed and would not mind putting it to her if she's behind it.

Neither of us are really technical Thanks for the help.