First of all; Neilan isn't retired CIA. He's been ''exiled'' here. Control has hinted that it might be because of a drinking problem (during the opening speech). But of course, it might be the other way around; if you were exiled to Zimbabwe, wouldn't you start drinking?
Second of all, if you make an ally out of him there is no immediate payoff in this serial. He is already friendly enough to provide you with all the info you need.
But if he becomes an ally he would become a permanent contact for the team, a contact owing much more loyalty to the players than to the Foundation.
If you can succeed a favor check and convince the Foundation to use its contact inside the CIA in order to have Neilan reassigned elsewhere, he could become a very useful contact. Especially if he's reassigned to Langley (CIA's HQ) until his retirement. Having him reassigned to one of the major European capitals wouldn't be bad either.
BTW; he doesn't need to be an ''Ally'' to be useful. With Luddite's AD he is already ''Helpful'' which isn't half bad. These dispositions only determine how far and how much risk an NPC will take to help (or hinder) you. An ''Ally'', for example, would risk his job and his life without hesitation. A ''Helpful'' NPC is still willing to go to great lenght but his loyalty is not unconditional.
But ''Helpful'' or ''Ally'', you absolutely need to get him out of Zimbabwe if you wish to use him as a contact. I have no plan to send you back to Zimbabwe once this business is done!