Tell your friend NOT to involve the Police at the moment, if her sister is not ready to leave,doesn't want to or is too terrified then it could make matters much worse for her. It's a shit situation, knowing what's happening but feeling helpless.
She could tell her DS what her own son said, so she becomes aware that her sis knows and wants to help, support, assist in escape or just be there to talk to.
The phone is a great idea - so long as she can safely hide it from him.
Tell your friend to get clued up about www.womansaid.org.uk - your friend could even call them to get all the info needed if her sister can't face it - if she decided to leave then the police can help her to leave safely if she tells them he's violent.
She could tell her sis to come on here and read about others in similar situations, she doesn't have to post (just remember to delete history if she does in case he finds out) There's masses of advice and many threads from inspirational ladies who've been in the same place.
he probably won't kill her
They don't always mean to.
The longer she stays with him the more she's at risk. The fucking arsehole abusers often get braver and more confident when they keep getting away with it. It especially sounds dangerous that he's taking steroids.