so what should i do ?

so as most of you guys know school has started back and my friend told me she started dating someone and i was all like good for you but then she told me is he our pe teacher, i honestly sat there for a minute trying to process everything then she continued to tell me how he invited her over to his house this tuesday. my friend is 14 and our pe teacher is 35 should i tell somebody i'm scared for her and i don't know what to do and i need other people's opinions on this also my friend is the type of person who doesn't like to listen to others so even if i say something she may ignore me so should i do something or leave her alone?