How can I help a friend to let her abusive boyfriend go?

Well she should dump him. You can help her the most by talking to her and point her to books, counselors, websites, about abuse. NOT by demanding she leave him, or calling the police on him retroactively. (because that will create a big problem for you and if they aren't breaking up, they'll defend their little couple hood from YOU who are trying to end what they both want)

Rather, she has to WANT to end it, and you can support her and help her in that direction and let her know that she deserves better and all that. MAYBE talk to her parents, but you know what? If she takes it from him, likely she takes it from her parents already (no matter how they seem on the surface) so that might backfire.

A school counselor or something like that might work out much better; plus a counselor could talk to her, police, parents at least as effectively as you.

Tell her parents. make her to understand that it's not good to stay in a abusive relationship. dump his ass. and what ever you do doesn't work, sometimes people need to live something, to wake up! but let authority ( police for example) and the family knows. that's physical abuse!

Tell her 2 dump his disrespectful ass on the curb!! its only gunna get worse and if she thinks she can "save him" or "help him change" SHE IS WRONG!!!! you can't change a guy...no matter what she says she cant...he f***ing slapped her...he deserves an ass whipping...an abusive relationship never gets better...only worse...trust me...my sis was choked and beaten...and she still THOUGHT she could save him...it was a joke 2 spend 3 years n that relationship.

Tell the authorities...he has no right to lay a finger on a girl...dump his ass. Period.

LET HER KNOW You R HER FRIEND AND You WANT THE BEST FOR HER. SHE SHOULD REALISE SHE IS BETTER THAN THAT AND THERE'S MORE GUYS OUT THERE WHO WOULD APPRECIATE HER AND GIVE HER KISSES ON THE MOUTH RATHER THAN SLAPPING HER. SHE IS STILL YOUNG AND DOESN'T NEED TO GO THIS ROUTE SO YOUNG. A GUY LIKE THAT WILL GET HER NOW WHERE! SHE SHOULD SEE THAT HE DOESN'T RESPECT HER IN THE FIRST PLACE BY GOING TO A PARTY WITH A GIRL WHO HAS A CRUSH ON HIM, AND THEN HITTING HER. TELL HER You R TRYING TO ONLY BE A GOOD FRIEND AND HELP HER OUT WITH THIS SITUATION AND IF You WEREN'T A TRUE FRIEND WHO CARED FOR HER You WOULDN'T WANT TO HELP HER. BUT MAKE SURE You LET HER KNOW. You R TRYING TO HELP HER BUT You CANT MAKE HER DECISION'S SO You CAN ONLY ADVICE HER BUT NO MATTER WHAT HAPPENS You WILL BE THERE FOR HER. :)

You have to tell an adult. Her parents, or a guidance counselor at school. It will hurt her now, because she is in love with this douchebag, but it will be better for her in the long run.

I don't know what action the parents would take, but they might ban her from going out with this guy, or contacting him. Or they might ask the parents of the guy to leave her alone. I don't know. But right now, being her age range, she will not listen to ANYONE. Teens at that age range think they are right about everything. I think the only people who can control this is her parents, and the school.
Sit down and talk to them, without her around. If you are afraid it will ruin the friendship, then ask them to conceal your name regarding this issue. And explain that you are just worried about her, and care about her well being.

tell her she deserves better because she does.

Well she needs to realize that she will find someone who will treat her right. As for him slapping her in the mouth. "oh no" that wouldn't fly. You need to tell her that what he is doing is wrong and make her realize that what if it was you in her position what would she tell you. (does that make sense? ) Maybe that will help her realize a little better.

How can she not want to dump him? I mean yes she might love him and say things like, he was just in a bad mood or that she blamed it on herself but really. you must try and show her a lot he facts of what her boyfriend really is and then you have to just wait, you can't force your friend to do anything but you must be there for her all the time every time she needs you. she will eventually realize whats going on and she will leave him,

I would dump him because that's abuse and she shouldn't have to deal with anything like that. she can find way better. just be there for whatever she decides to do.

If this jackass is already hitting girls when he's a teenager, imagine what he'll do 10 years from now. I'd guess that he's getting hit at home and I would get an adult involved to check into things. It could change their whole future if he/she get help.

If it gets worse, tell her parents. Abuse escalates and it's a terrible thing to get into so young (at any age, really).

You need to tell here it's not healthy to stay with someone who abuses you. I know it's just once, so far. But rarely does it stay as a one time thing. Trust me it starts a push then a slap and soon your friend will find she is a punching bag for all his frustrations. Please do your friend a favor and get her out of this situation or at least support her, because when it's over she will need your friendship even more then she does now.

You need to back off and don't get involved in her dramas. She is young and is in love with him. She will dump him when she will have enough of him slapping her. I would dump him if he had slapped me in the mouth because I will not put up with that crap anymore.