If the ends do not justify the means, how do they here?
means = lying, a violation of the 8th Commandment
(She lies that she is a minor who had sex with a 27 year old.)
ends = revealing a truth about the abortion industry
By your reasoning, all undercover policemen are committing sin. 
Are they lying because the pretend not to be policemen?
They may be lying but that’s not necessarily the same as committing a grave sin.
Further, using your original example. If someone lied about her age in order to prove that the abortionist was already breaking the law, this would not be the same as a girl lying about her age to procur an abortion.