Help me nabbing a cheater

my friend is being cheated and she is doing it 2nd time.

Is it possible to know from an IP address, that what other email accounts is being accessed from that IP or computer ?

We know her current email ID' password, so could know its IP from 'last account activity'. I think sheis using another emailid (most probably gmail) for chatting or emailing.

Is it possible. I am a NOOB and non-techi.