Request for Reference from UK

If I were you, I should write back to the company requesting for an authorization letter from the ex-employee as it is regulated by the privacy policy in Hong Kong to get authorization before disclosing the private data of anybody in Hong Kong.

You can do it in another way round writing to the ex-employee or call him/her to get the written authorization instead.

In case you are not going to reply the mail, the reputation of your company will be affected and it is not a common practice in human resource management. Also, your current employee may know the company will do the same if the ex-employee tell other colleagues who is currently working in your company.