Not only Old World monkeys, but animals even as big as elephants are struggling with loneliness at Latin American zoos. Take Sandro and Koala, the two Asian elephants whose companions died this year as examples. Other than those, I'd like to mention Luna, the white rhino.
Yes, when I mentioned those, I wanted to involve all the species suffering with loneliness in zoos nowadays. I think the quality of the handling of species would help to solve this problem. Zoos could cooperate more when it comes to this. For example, joining elephant "Sandro" with "Koala", and other asian elephants we have in Brazil; all of them in Rio's new exhibit, wich seems to become the best for the species in the country when it's ready. Not even necessarily talking about forming a reproductive group, but simply providing the animals a nice interactive life.
Wouldn't include "Luna" as a very urgent case, not as much as elephants for example. Rhinos not necessarily live in groups. I don't know what are the plans for her, nor for rhinos at BH when she dies though.