MY FRIEND IS ABUSED! PLEASE PLEASE PLEASE help.?

I need advice, real advice...I called some teen hot line but they have to say certain things and I got scared and hung up. My friend elle is 13 and her dad beats me she just admitted it to me, she didn't openly talk about it with me either I saw him do it. Well I saw him slap her across the face. I didn't really hit me until after that, that she always has a sweater on or there are always fading bruises on her arms and legs and chest and neck when she doesn't have long pants or a sweater on. Elle always always misses school the legal limit is like 20 unexcused and she missed like 18 unexcused and i have no idea how many excused. So I was texting her and we had this really really long convo and she finally admitted that he abused her like badly abuses her to with belts and hangers and stuff and that's why she always misses school and is breaking stuff....but she made me sware sware sware not to say anything she said she lived without telling for 13 years so I have no right to tell and that she doesn't want to talk about it ever again..but its just so unfair she is the sweetest girl and shes so popular and everyone loves her she doesn't deserve to beaten and she told not to ever bring it up again. what do i do?