Since it was something that "several people" have seen, you could always warn her yourself, but you certainly would have to consider the fact that you husband would likely lose his job. You could write an anonymous note and send it to her, perhaps suggest that she consider hiring a private detective to catch him in the act. I don't think your husband should get in the middle of it, guy or no guy.
I welcome your thought, let me know if you want to talk more
There is always the risk that you will lose your friend. Mainly because if she loves him, she wont WANT to believe that it's true and if he flatly denies it and she has no proof them she'll be more inclined to believe him.
You can let her know in the letter that you hesitated to send the letter because you were afraid she would think its a hoax. Urge her not to run to him with the letter but to do her homework, and find out the truth for herself.
You would want to know...I would want to know...but believe it or not, there are some people who would prefer not to know. Sure, some might say that you're sticking your nose in, but in my opinion,as a friend, I wouldn't be able to let it go.