True!They should have persevered with it, though trying to add strange females to a pair with a baby isn't the best time- the male would be delighted to have additional females I'm sure, but not the female. I also think lack of females to distract him is why the younger male abuses infants like that, he has no other focus of attention. In a big troop this would less likely to happen I think.
This sounds very much like a case of husbandry inadequacy and not heeding the social organisation and structure of natural Hamadryas groups.
BTW: the zoos of Emmen Netherlands and Hamm in Germany have definite surplus Hamadryas. The latter even direct 30 to give away ...!!!