Help friend confirm his wife's affair?

I asked this in Marriage & Divorce, but it's actually more about friendship. My best friend of many years suspects his wife of having an affair and wants me to help him find out for sure. I'm reluctant to get involved, mainly because our wives are best friends as well. They spend a lot of time together, travel together, and enjoy a lifelong relationship that I don't want to jeopardize. My wife has joked about her friend's wild streak and how she might be getting some on the side, but she also says that of other friends that aren't as close to her. Now he's asked me to ask my wife if she knows anything and report back to him. I've been putting him off up to now, but he's starting to put a guilt trip on me for not helping out a friend. I feel like this is a no-win situation no matter which way I go. I feel like if I help him with this it will affect my relationship with my wife and her relationship with her best friend as well. Is it better to refuse to help, or try to get answers for him?