How to get a Facebook Password?

[ashley]

Please hear me out. I pretty much know my mother is cheating on my stepdad. I have some proof, but it's not enough.. I need her facebook password to get their messages to eachother. I know it's terrible but my stepdad is so broken up, knowing, not knowing.. it's killing him. So I think giving sufficient evidence will allow him to move on faster and leave her. If anybody has anyway I can get her password, please let me know. Thanks.