hmm i didn´t see one really useful advise in this thread ...
sure she´s cheating on her husband and yes i would tell it to him in most cases
BUT
if i know he is a violent piece of shit I wouldn´t tell this fuck anything except
. talk to your friend and convince her to get a divorce BUT she shouldn´t be alone when she tells her husband.
ask some friends ( males! ) to stay with her when she is talking to him. or better they should meet in a public place. and i´m sure if he knows about this fuck with his friend he will give a divorce hence...