A little tl;dr sorry

Fuckkkkk.
I don't know what to do.
Yesterday, my friend asked me to babysit with her and I told her I couldn't. I have every other time but I couldn't this time because I had an essay and some major studying to do.I had a feeling something bad would happen and that I should go, but I ignored it and toold myself she could handle it.
The kids are 8 months and 5 years old. When we go, usually she looks after the 8 month old and I look after the 5 year old because he doesn't really like her. So around 8pm she called me in tears saying the 5 year old (Tyler) wouldn't listen to her and was just being really fussy. I talked to him, told him I was sorry but I'd be there next time and asked if he could please be 'a big boy' and do whatever she says and try to be helpful since his brother has a cold. I thought that would help, but apparently it didn't because he was still being fussy when the dad called.
Before I continue, let me just say when I met the dad he gave me a creepy vibe.
Can you see where this is going?
He asked how everything was going and if she got the kids to bed yet. She told him no that Tyler wouldn't listen and he was running around the house screaming which, in turn, was causing Ryan to scream. So the dad said ' Alright, I'm coming back to help you put them to bed, don't worry.'
I don't know the specifics of what happened next but the point is, he pretty much forced her to make out with him when SHE DIDN'T WANT TO.
She called me after he left crying but wouldn't tell me why. Called me again this morning and told me she'd tell me when she got home. Called me when she got home and said it was too hard and to call our other friend. At this point I was worried , because, I thought it had to do with her Grandpa who's in the hospital.So I call our other friend who tells me and then call the first friend back.
Problem is...she won't tell anyone and she said she'd probably go back if the mom asks her to. She doesn't wat to tell the mom because she doesn't want to break up the family and put the kids through that. I'm telling her that if she doesn't tell someone...then who knows..maybe he'd do it to the kids. Slim chance..but still.
What do I tell her?
What should I do?
I want to tell her mom, or my teacher but I don't want her to get mad at me.
Help?