Any real chance he did NOT cheat?

My sister's husband was just diagnosed with herpes this past week. He had a very bad outbreak and this has never happened before. They have been married for 4 years and were supposed to be monogamous for a year before that. She was already contemplating a separation for a variety of other reasons and then this happened. She told me she does not believe he cheated on her because he said he didn't. She will get her test results in a couple of days and has never had an outbreak (but she does get an occasional cold sore when she gets sick) . I just cannot believe that he is innocent in this matter. IS there ANY way it was dormant for that long (5years)? I couldn't stand him before this and now I don't know how I can help her if she wants to believe he has been faithful when it should be the last straw in my book. Any advice or knowledge would be appreciated.