She has it but he doesn’t ?

I have this friend who has been dating a guy for a couple months now and they were happily falling hard for eachother . She recently just told me that she got her test results back and she tested positive with a sti. She told the guy she was dating but when he got tested he told her that his results came back negative. I think he’s lying but she doesn’t . She’s just blaming herself because now she’s assuming she got me from her ex . How could they be having unprotected sex and he not have it ? Is he lying ? . Someone help me to tell my friend what to do .