She has a right to know. And she needs to know, if only to protect herself from disease. However, make sure you have solid proof, not just rumors or suspicions.
If you are absolutely sure, then yes, you must tell her. It is not your place to protect her; let her decide how much it bothers her on her own. If it breaks up the marriage then the marriage wasn't good for her anyway. And if you merely suspect it, then you need to tell her what makes you suspicious.
Although it will be painful, you have to tell her. Be sure that you have proof, however, or she might not believe you. The husband is, at the very least, putting her at risk for a sexually transmitted disease.
Confront him first, tell him that if he doesn't tell her first, you will. She should know, she's at risk for any STDs he could get from whoever he's with.