what should my friend do? and does she have a case?

my friend has been working for this company for five years... In this last year she has started to move up in the employment ranking. Her job causes for her to fire and hire lots of people on a regular basis. Two weeks ago she fired this guy whose girlfriend also worked there. Upset about her boyfriend being fired the girl filed a sexual harrassment complaint on my friend. My friend is a lesbian. She has worked there for 5 yrs and has a clean record. she has never been in trouble, never been written up, and never had any type of complaints. but the company fired her. What should she do. Doesn't she have to right to defend herself and prove her innocents. What kind of investigations do companys do to prove the alligations are true.