Wow, so many different answers :yuno: I think.. just my opinion, tell the husband, i mean think if it was you and he knew your wife was cheating on you and you never knew a thing and you thought she loved you loads for you to find out she'd cheated on you. I mean dependant on how long they've been together will change it from being a major heart brake to something not as big. I mean, if they cant be honest with eachother then whats the point in being together? The whole point of a relationship is being happy and actually loving one another and the main factor of it, is 'TRUST' without trust, it wont work. I think the best option is find out a bit more, 'somehow' but gotta be sneaky. And if your certain, tell the husband and see how he feels. If he cant speak to his wife about, ask if you should speak to her next time she asks for help.
Just my opinion.