If he's got no service and he's given them ample opportunity then they don't have a leg to stand on.
He needs to send them a recorded delivery letter telling them of the issues and that he wants them fixing by a set date (give them two weeks). In the letter state that unless they are fixed (on a permanent basis) by this date then he will be ceasing payment as the service isn't up to scratch in accordance with the
Supply of Goods and Services Act 1982. In the letter state that if the service isn't fixed he considers any further obligations he has to them financially to be over.
Either they'll fix it, or they'll not. If they do then he really can't just get out of the contract. If they don't then consider it over and wait for them to try and chase him for the money. If the service stopped working and they haven't fixed it in a reasonable time I'd like to see them try.
---------- Post added at 22:32 ---------- Previous post was at 22:29 ----------
Also if he wants to move ADSL provider his current company MUST give him a MAC so he can leave them, them saying he owes them money is not a sufficient reason to refuse a MAC.
That way he can fight it out with them after he's moved rather than be trapped for months.