It looks a little like mange to me. The kittens will need to see a vet for a checkup and to have their tails looked at. You said mumcat took one of the kittens back - are you keeping them outside? Even though it's summer where you are, they need to be inside, isolated from any pets that you...
Her best bet may be to use a drop trap in the area he was most recently seen. She could call him, shake a tin of treats (whatever noise it is she used to use to call or feed him). This thread talks about how to make a relatively quick and inexpensive drop trap...