It's possible that your friend's concerns could be considered as a form of paranoia or anxiety. The fear of the woman's husband showing up at his door may be a result of guilt, worry about potential consequences, or a sense of moral conflict in being involved with a married person.
Having concerns about the husband's reaction is not uncommon in situations like this, as it involves complex emotions and potential risks. It's crucial for your friend to reflect on the situation and consider the ethical implications of being involved with a married person. It's generally considered morally wrong to engage in relationships with someone who is married or committed to someone else.
If your friend truly cares about this woman and respects her, he should encourage her to resolve her marital issues directly with her husband, rather than continuing their involvement while she is still married. Encouraging her to work on her relationship or consider a separation/divorce if needed is the more responsible and ethical approach.
If your friend's concerns are causing significant distress, affecting his daily life, or leading to obsessive thoughts, he should consider seeking professional counseling or therapy to work through his feelings and emotions in a healthy way. A mental health professional can provide guidance and support in handling such complicated situations and help him gain clarity and make healthier decisions.
Keep in mind that as an outsider, you can offer support and advice, but ultimately it's up to your friend to take responsibility for his actions and the impact they may have on others involved. Encourage him to make choices that align with his values and consider the well-being of everyone involved.