Help!! my best friends boyfriend is...?

My best friends boyfriend is sort of cheating on her..We are thirteen but anyways nobody could get a hold of her her phone was out and she doesnt get cell phone service at her house.soo Lets call her boyfriend Josh and her Brittany.Josh kept asking me if id heard from Brittany and I said no no ,then he said he might be breaking up with Britt, so i was messing on myspace and i got on his page and he has i love you (his new girlfriends name) plastered all over it so anyways me and britt havent talked much cuz of her phones but she found service and called me and were going to the movies saturday. So should I tell her or let Josh?? Also, would this be cheating on her since he couldnt get a hold of her to break uo with her??