American-Japanese Dual Nationality

[Autumnal 25]

Could someone help me conclusively answer the following question? Family friends have received greatly conflicting information regarding the citizenship status of their 14-year-old daughter, and I would like to help them clarify any actions they need (or do not need) to take.

My friends, both American citizens, were stationed in Japan when they had their daughter. They were married; I am not sure whether either was a Japanese national at the time. Her father registered her birth on base/with the military and the proper paperwork with Japanese officials. Authorities conferred American citizenship and Japanese citizenship on her. She received an American passport and now lives in the U.S. This year, my friends received a letter from the U.S. government notifying them that their daughter would need to choose whether to be a Japanese citizen or an American citizen at the age of 18. She could not maintain dual nationalities. They'll let her pick once she comes of age.

* Is this an American or a Japanese policy? (The State Department suggests she would not have to renounce her American citizenship: http://travel.state.gov/travel/cis_pa_tw/cis/cis_1753.html. The American embassy in Tokyo doesn't imply this has to happen either: http://japan.usembassy.gov/e/acs/tacs-7118.html)

* Because she automatically obtained her citizenship at birth, by American laws, is this a Japanese law?

* Does she have to renounce a nationality? (The Ministry of Justice in Japan: http://www.moj.go.jp/ENGLISH/information/tcon-01.html - a bit unclear.)

* If she claims Japanese citizenship, is she automatically assumed to lose her American citizenship, or could she still travel into the U.S. on a U.S. passport, pay taxes as a foreigner abroad, and whatnot?

Thanks!