Is it legal for a U.S. citizen to receive/use diplomatic passport of another country? (not for entering/exiting the USA)

Here's the situation:

My friend was born in the USA while his parents were sent here to work in a company. After he was born, the parents applied and received U.S. passport. When the parents were transferred back to the home country, he went back with them. At this time, he also automatically received the home country's passport.

Fast-foward to present day. His parents work for the home country's gov't and are now sent to a third country as diplomats. They obviously are using the home country gov't issued diplomatic passports. Since my friend is still in college, at age 19, he was also automatically issued a dependent diplomatic passport.

Now my friend have both a US passport and a foreign diplomatic passport.

Obviously wheneve he travels back to the USA, he uses his US passport. (but he does carry both passports with him most of the time) However, when he goes to visit his parents in that third country, he presents his diplomatic passport.

Does the possession of the foreign diplomatic passport put him at any danger as far as his U.S. citizenship is concerned? This may be a moot issue in a couple of years, as his status for dependent diplomatic passort expires when he reaces the age of 21. However, this does not change the fact that at one time he did have a foreign diplomatic passport.

Any information appreciated.