In the UK, an employee usually receives his wages after income tax and National Insurance contribution has been taken off. This means that you can spent the last penny if you want without worrying about the tax bill later in the year.
Your friend will need to apply for a National Insurance number from the Department for Works and Pension once he arrives in the UK -
Also things are different in the UK from Hong Kong - for example people in the UK don't usually go and see the doctor if they have the cold (感冒) - the doctor will tell you that there is nothing he can do - all you need is to drink lots of water and rest.
Wish him the best of luck!