Not sure where you're going with this, but the diplomatic mission that this person claims to be affiliated with can confirm or deny that this person is indeed a "diplomat".
It's rare that a diplomat is not a national of the country they represent. The Vienna Convention on Diplomatic Relations says a person should be, "in principle." It can happen, but it's not common.
I'm just speculating, but is this person claiming immunity in some form? If so, bear in mind that under the general heading of "diplomatic personnel" are a variety of different categories, from diplomatic agents to service staff, and different rules apply to different types of personnel.
Hope this helps!