Possible to prove who's broken into a yahoo email acct??

My fiance's ex-wife (we believe it to be his ex anyway) has broken into his yahoo email account. How can he go about proving it was her? (Esp if it was done from a public place, like a university library) Is there a way to find out the IP addy that the change was initiated from? And finally have any of you ever heard of someone having charges filed against them and a judgment actually being given for "hacking" into a private party's email account for the purpose of snooping?

Thank you so much for your help!