We bred horses for 40+ years.
99% of all rank studs before they are bred, in hand, or pasture, are worse after the breeding(s). NO way would I turn my horse in with him after he's had his way with the girls. He must be mannered first. And the job of mannering him after breeding him anyway, any place, will be dangerous for whoever does it.
Pasture breeding one of these is the worst thing you can do. He is being taught that he can have them any time his little heart feels like it, and that carries over to wanting it the same way, any time he is around, or in smelling distance of, any mare, while be worked in hand. And he'll do just about anything to get loose and go after the mare. Now what if that mare that he get's loose, to go after, has a child riding her. One dead kid.
Your cousin's behavior borders in true insanity. Would he put a loaded 6 shot revolver, loaded with 5 shots, up to the head of the child and pull the trigger 5 times. Cousin had better be ready to serve some time in the big house.