Catching a cheater.

So, my best friend is a married man (I'm aware that many don't approve of this). The other day he received an anonymous phone call from a woman saying his wife hasn't been as faithful as he thinks. Now I've suspected for a long time now that she was cheating and now he wants to try and get evidence against her. Is there any way someone could think of to catch her doing something or figure out the names of the past ones? He doesn't have access to her phone. If he even touches it she goes bat shit on him.