1. He was told in 2008 he must issue Court proceedings to make a claim. He hasn't done so. There is no debt.
2. If he does instruct a firm they will be debt collectors (although some debt collectors are part of solicitor firms & may go under a solicitor's letterhead). Their first contact is almost invariably by letter. You can either ignore it, or write back (recorded delivery, keep a copy) telling them the facts - i.e. you dispute that there is a debt & why. Ask them to return the case to their client.
3. If anyone is sent to see you (very unlikely) they will not be bailiffs (although they might try to pretend they are) because a CCJ is essential before bailiffs can be instructed. They will simply be debt collectors who have no rights & no powers. Do not let them in. Tell them you do not owe any money & they are to leave. If they display any aggression tell them you will ring the police.
4. Unfortunately, there is nothing to stop him instructing a succession of debt collectors. If he does, at some stage this could come under harassment legislation. CAB should be able to advise you on the implications.