A close friend family friend was a very senior diplomat - is it appropriate to have a letter from him?
In general, it's not helpful to get recommendations from celebrities (unless their celebrity is from being a star in your field). The letters should be backing up the claim that you are capable of doing PhD-level work and thriving in a program of the target school's caliber. This is why it's usually a bad idea to have non-PhDs write letters - how can they attest that you are capable of doing something, if they haven't done it themselves, and thus have no real frame of reference for what it requires?
If your senior diplomat has a PhD, it might be reasonable to get a letter from him.
I'd say that you want at least one letter to come from your field. Have you done research with a political science professor? Even though you are not a poli sci major, have you taken any classes in the subject? You might be able to get an in-field letter this way. Beyond that, you want people who will attest to your capabilities in relevant skills, such as research and writing. These skills are needed across fields, and so it's not unreasonable to have profs from outside of your field write them if those are the profs who know your skills.