The case rests on whether the Immigration Department judges him to be "doing business" here or being "employed". If he can make a case that he is genuinely employed in the UK and just working on a project in HK then there will be some flexibility. But if he is just a sole trader simply selling his time and expertise (i.e. he isn't selling you something that his employer has made and he is just installing it) then it would be very difficult to persuade ImmD that he isn't "employed" in HK.
The onus is on you as the employer to ensure that what you are doing is legal, so if he gets caught out both your company and he are liable for penalties (in the case of the employer it is a fine of up to HK$150,000 plus one year in prison).
If I were you I would get you "employee's" opinion verified by ImmD.
http://www.immd.gov.hk/ehtml/topical_8.htm