Parenting advice

I've been with this guy for 3 almost 4 years and he has a 12 yr old daughter and we have a 5 month old together. Well his parenting before hand wasn't so bad (first year in) then as the years went by his daughter became more and more disrespectful and is now smoking weed, cussing, and doing other things that a 12 year old shouldn't be doing. We get into arguments about his parenting because he takes it easy on her and doesn't do anything and she ends up doing whatever. He used to whoop her and now he claims that he is afraid to go "too far". He takes her phone away just to give it back in a few days and then she is right back where she started. I've talked about how he needs to man up and be a parent and discipline his child because I don't want my son to be the same way and I've told him that I'm going to be the one who raises our son (teaching right from wrong) because he fails to do that with his child now. He says that he is going to do things differently this time around but that to me is like saying fvck the one I already have and I can start over. What is something that I can do to help him out? Or something I can get him to do? I don't want to not help, but it's like he doesn't care what road she is headed.