I found out my mates little sister had sex! should i tell her?

My friends little sister is 13 and has been going out with this boy for 3 years! Which I dont think it is right and I think they are too young! They are properly in love and they are quite mature. but i was stood in the lunch queue and the boyfriend was infront of me and he said to someone who he was with 'guess what! Me and cathy ( thats her name) did it!' his friend said @what? you mean you did IT!!!!?' and the boyfriend nodded. I dont know if he was just lying but it isnt something you lie about. Should I tell her? or is it none of my buissness. If i am wrong, i think she might be a bit annoyed at me