We had the same prob just a week ago. Our drake was trying to mate with the chickens also and very mean to them. We gave him away and the female duck since she was raised with the hens and rooster also is doing fine on her own the rooster is very proud to have her with his flock also. i watched her close last week incase she got lonley and she is just fine. ( I think it was cause they were all together as babies) 
