I really feel for you because you are in an unpleasant situation not of your own making. It's one of those times where you could say you wish you didn't know. But what exactly do you know? Well, precisely, nothing. You have heard rumours. A rumour is not a fact so if you were to talk to your friend, what exactly would you say and would she believe you? If these rumours are malicious and there is no truth in any of them, you will either become the villain of the piece as she will turn on you or you will be responsible for ruining her relationship with her husband as she will automatically become suspicious and jealous.

If all you have heard are rumours, then you can quite legitimately ignore them and keep quiet. Should your friend become aware that you knew, your defense is quite simply that you did not believe this malicious gossip but make sure you don't discuss it with anyone else.

Be aware of the devastation you would cause if you revealed your suspicions. Even if they were well-founded you should think twice about this. A messenger bearing bad news is never very welcome and you could lose your friendship over this.

If you had concrete proof that her husband was cheating then you would have to bring it to her attention but more by letting her discover it than telling her outright. Whatever you decide, tread carefully!