A voice-activated recorder hidden under the front seat of his car would turn up any phone conversations, but I'd check first to see if that's legal in her jurisdiction before I'd ever advocate that, and even then I'd probably have a trusted friend or family member listen to it first to protect her heart and emotions, as it can be BRUTAL.
Might be simpler to just operate from the basis that there likely IS an affair going on, and proceed accordingly. Certainly has all of the signs, and that way you're not devastated if one does turn up.
