Should she stay ?

A girl i go to school with recently found out that she has Chlamydia. She says that she only has had sex with with her boyfriend of 2 years. She said she told him & he said a few weeks ago he got tested & he was clean. She feels that he's lying and that he gave it to her, but she's head over heels for him. So, if he did give it to her should she stay or should she go? & what steps should she take?