Problems with a bad ISP?

I'm asking out of curiosity for a friend's experience with a bad ISP. The internet service was terrible, and would often drop out and customer service was often unavailable to fix the problems(closed on weekends and the lines were sometimes closed earlier than advertised). He called multiple times to complain about paying for a service that was often not provided, until, during a particularly long period of no internet, he finally told them that he would not do any further business with them until they fixed it. If it were not fixed by a certain date, then he wanted the ISP to cancel his service. It went unfixed, so he ended his business with them. Now they are pestering him for a cancellation fee, threatening him with debt collectors. He is adamant that he will not pay any more fees for a terrible unreliable service that did not provide what it said it would. What happens after this? They haven't replied to him in a while. Would they have to tell you when they've put debt collectors on you? I don't really understand how credit ratings work, but could his credit rating be adversely affected without him knowing or having a chance to argue against it first? If it came to it, would the law (ACCC or otherwise) side with him?

So I'm asking, what would you do?