Help my sister leave him...

My sister is 31, almost 3 years ago she started dating a guy (35ish??), let's just call him Z. There were some red signs from the get-go. He has 3 kids from an ex, their relationship is very bad. From name calling, to her pushing him and scratching him, yelling right in front of the kids etc. The point is, the ex is fairly unstable.

He is a coke-addict, sober for the past few months, trying to get his life on the right track. He works on and off, skipping when he doesn't feel like going in, then panicking about not having heating in the house and my sister is left to try to figure out the solution. He does name call her when he is angry and gets furious with her over his own stupid decisions. His oldest son was diagnosed with cancer a few weeks ago, which is of course a huge stress source, but since it's been found that the type he has is 90% fully treatable, and he is responding really well to treatment. The biggest problem in the situation is that he is extremely selfish and verbally assaulting. He is into racing (he races himself), and despite not having money to spare he spent his last bit of money on entry fees. The day after he had to bring his son to his weekly chemo and wanted my sister to get out at a petrol station and give him cash for parking fees. She said she'd have to go to the ATM because the petrol station would only give 50 notes and she doesn't have that much on her card. He threw a huge hissy fit over how she doesn't care and that he has no time to go to the ATM before the appointment. Dropped her off at work, then left. He started then texting her, calling her a cunt, a bitch and all sort, then she told him he was being selfish wasting the money when he knew they needed it next day... He started panicking when he arrived and begged her to pay for the parking through her phone - which she ended up doing because the oldest kid would be very sick after his treatment and didn't want him to suffer his father's decisions consequences of having the car clamped and having to take a bus home... As soon as she did so, he started telling her how she is the best and he loves her etc. The next day she came over and told me he was so very sorry about his behavior, but the thing is this happens way too often. She expressed a few times that she wants to leave but she is scared that Z goes back to taking drugs and the kids suffering the consequences... he also throws in her face that she needs to leave him alone because his kid has cancer... he is using it a lot as an excuse for his shitty behaviour. AIBU to tell her she needs to get out asap? It is a verbally abusive relationships and I can see it turning into more... I will show her the replies to this thread, hopefully it will help her come to a decision.