We had a boiler with the flue 1 foot above the ground under our rear garage window, which broke all the regulations. Was mentioned every year in the boiler servicing that the flue doesn't meet current regs, which would be a problem if we replaced the boiler or sell the house. But as long as the current boiler kept going, then having a flue breaking all the rules was obviously fine if it pre-dated the rules.
So I guess in this case there is absolutely nothing the current neighbour will have to do to comply until he sells up, or replaces boiler.