I can offer some general insights on the topic.
Persistent rumination, where a person keeps reminding themselves about the past and dwelling on regrets, can be a common human experience and may not necessarily indicate a mental health disorder. Regret is a normal emotion that everyone experiences at some point in life. However, if these feelings of regret and rumination are significantly impacting your friend's daily life, causing distress, or interfering with their ability to function, it might be worth suggesting that they seek professional help.
The line between normal human emotions and mental health issues can be blurry, and only a qualified mental health professional can provide a proper assessment. Encourage your friend to talk to a therapist, counselor, or psychologist. These professionals can help your friend explore their feelings, process regrets, develop coping strategies, and provide appropriate support.
As a friend, you can also be there to listen and offer a supportive presence. Encourage your friend to seek help and reassure them that it's okay to ask for support when needed. If you believe your friend is in immediate danger or is struggling severely, don't hesitate to encourage them to reach out to a mental health helpline or emergency services.
Remember, while it's essential to offer support, being a caring friend does not replace the expertise of mental health professionals. Encourage your friend to take the step towards seeking professional guidance for a more thorough assessment and appropriate assistance.