As with every conceivable issue regarding U.S. Passport and Visa regulations, this is covered extensively in the Foreign Affairs Manual.
First off, it depends on how old this traveller is, and who they are travelling with. For instance, 7 FAM 085 U.S. PASSPORTS AND VISAS AND DUAL NATIONALITY states:
a. Section 215(b) of the INA (8 U.S.C. 1185(b) and 22 CFR 53 require
U.S. citizens to enter and depart the United States on U.S. passports,
with limited exceptions.
d. See 9 FAM 40.2 Notes, 9 FAM 42.12 N4 (2)(b) and 9 FAM 42.71 N5.
What are these "limited exceptions", you might ask? 9 FAM 202.1-2 has you covered.
Applications for Visas for Certain Dual National Children:
(1) You should advise parents who apply for visas for dual national
children that regulations prohibit the issuance of a visa or other
documentation to a U.S. citizen or national for entry into the United
States as an alien. The children of foreign government officials,
however, may use their foreign passport for entry into the United
(2) After the U.S. citizenship of a child has been determined by a
citizenship officer, the consular officer may, to avoid delay or
difficulty, give a written statement to the parents for presentation
to carriers or immigration officials. The statement should make clear
that the bearer of the foreign passport is a dual national child of a
foreign government official or employee who is traveling to the United
States on official business and as such may enter the United States on
the foreign passport as an exception to the provisions of INA 215(b)
regarding valid passport requirement.
(3) A child under 12 years of age who is included in the passport of
an alien parent in an official capacity may be admitted if evidence of
U.S. citizenship is presented at the time of entry. A determination of
the child’s citizenship should be made by citizenship officer prior to
departure from a foreign country and the parent should be instructed
to have evidence of such citizenship available for inspection by the
admitting Department of Homeland Security Officer.
So, if your friend is travelling with his parents, and they're representatives on an official assignment, he can ask a consular officer for a letter stating such.
If, however, he just wants to avoid being identified as a U.S. citizen as he leaves his country, he is out of luck. It is illegal to issue U.S. visas to U.S. citizens.