Health insurance in Germany for a foreigner. (Munich)

My American friend has been living in Augsburg for two years. He retired in the U.S. He is not married. He never worked here. He has a German residence permit (Aufenthaltstitel) until next year.
His American insurance company says they will no longer pay his medical bills here because he has been gone too long from the U.S. Can someone please recommend where he can buy health insurance in Germany? Thank You for your answer.