Gmail didnt work for me after i tried it.
I think it does half to do with your firewall. Get the user to try webmessenger
http://webmessenger.msn.com
It works for me.
Notes about msn messenger signing you out:
1.It happens alot when you first start the conversation (Are the first to send the message)
2. One factor seems to be if the other person closed their window
3.Msn somtimes hates you and signs you out right when the most important person ever is about to ask you an important question