I'm not sure Immigration or IRD would see it that way. If they intrepret him as residing in HK (as in it is where he ordinarily lives, doesn't have a place back home and spends the majority of his time here, whether on a residency visa or not) he would be considered working here, illegally, if he doesn't have the right visa. When giving advice on these matters where people actually can get fined large amounts, kicked out of the country or in extreme circumstances, instead of making such absolute statement of fact, as in "If he is working for someone from back home (ie in the UK) then it's not illegal." it may be more helpful for the poster if you give both sides of the what if. In the situation he has described I think that he would be considered to be residing here.