no, not authorized
Specifically, 10 USC, Subtitle A, Part II, Chapter 45, Sections 771 and 772.
Section 771 states:
Except as otherwise provided by law, no person except a member of the Army, Navy, Air Force, or Marine Corps, as the case may be, may wear -
(1) the uniform, or a distinctive part of the uniform, of the Army, Navy, Air Force, or Marine Corps; or
(2) a uniform any part of which is similar to a distinctive part of the uniform of the Army, Navy, Air Force, or Marine Corps
Section 772 lists some exceptions:
(a) A member of the Army National Guard or the Air National Guard may wear the uniform prescribed for the Army National Guard or the Air National Guard, as the case may be.
(b) A member of the Naval Militia may wear the uniform prescribed for the Naval Militia.
(c) A retired officer of the Army, Navy, Air Force, or Marine Corps may bear the title and wear the uniform of his retired grade.
(d) A person who is discharged honorably or under honorable conditions from the Army, Navy, Air Force, or Marine Corps may wear his uniform while going from the place of discharge to his home, within three months after his discharge.
(e) A person not on active duty who served honorably in time of war in the Army, Navy, Air Force, or Marine Corps may bear the title, and, when authorized by regulations prescribed by the President, wear the uniform, of the highest grade held by him during that war.
(f) While portraying a member of the Army, Navy, Air Force, or Marine Corps, an actor in a theatrical or motion-picture production may wear the uniform of that armed force if the portrayal does not tend to discredit that armed force.
(g) An officer or resident of a veterans’ home administered by the Department of Veterans Affairs may wear such uniform as the Secretary of the military department concerned may prescribe.
(h) While attending a course of military instruction conducted by the Army, Navy, Air Force, or Marine Corps, a civilian may wear the uniform prescribed by that armed force if the wear of such uniform is specifically authorized under regulations prescribed by the Secretary of the military department concerned.
(i) Under such regulations as the Secretary of the Air Force may prescribe, a citizen of a foreign country who graduates from an Air Force school may wear the appropriate aviation badges of the Air Force.
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This will be the exception used - the person in question was a member of an "amatuer theater" group at the state fair (blah blah blah) ...