No "force" involved.  The rules may be one thing, but being that s/he knew that there was CLEARLY an unfair advantage due to the hormone treatments, s/he should have done the "ethical" thing and sat out of the matches.  We all make choices in life.  Electing to change gender has consequences that come with it.  

Just because the "rules" state that s/he must compete in a particular group, if s/he knows that the match up is unfair, then this would be a "ethical" decision.  There may be things that are "legal", but not "ethical".  It's all a part of "good sportsmanship".