I'm writing for a friend who has recently admitted to to his ex wife that he read her messages to confirm she was having an affair. She claims it was illegal and he was stalking her, is threatening legal action. They were married 16 years, with children and had no apparent significant relationship issues. He became suspicious after observing behavioural changes including flirting with the man she had an affair with and using her phone secretively and more frequently. She passionately denied any infidelity and made him feel guilty about asking. Eventually he accessed her phone, having noticed her pass and remembering it. He found clear evidence of the affair. He then continued to read the texts and frequently access their exchange - including during a period she vowed to stop the affair and try to salvage the marriage. This stopped when she left the marital home, or earlier. Was this illegal - stalking or privacy breach perhaps? I'm surprised there is little discussion about this on the net as it must be common.