Hi sunshinefl, welcome to Psych Central forums. It's nice of you to reach out in an effort to help your friend. Sometimes an adult child can get involved with either a bad influence person or group and completely distance from a loving parent like that. If his daughter is mentally ill she may be more susceptible and may even have developed a drug problem. Four years and he has not seen her? Does he even know where she is? What kind of mental illness does she have?

Can you share more information so members have more to go on to better advise?