tofu

his hkid is invalid.  is his持特區護照 still valid, usually it is only good for 10yrs.   is your friend a canadian, he can use his canadian passport.
1964 或 1965          2004年1月12日 至 2004年3月13日身份證
for that period, you can use payroll or visa statement.  for the whole 10+years period, you can use t4 or noa.
they just asked me where was i, but didn't ask me to proof it.  because i think it is quite common

tofu

did your friend born in hk?  if not, they will request more doc. from your friend

tofu

"""是的﹐他在HK 出生﹐有出世紙。。。我猜最好帶埋﹐是嗎﹖""" - yes

also depends on how old is his hkid.  

my grandma hasn't been back to hk for around 15 years.  when she went to renew her hkid, they requested her to proof she had been lived in hk for more than 7 yrs(she was not born in hk).  they said her record was too old and not showing in the system?!?, also the hkid was changed few versions already.  so i think if your friend is holding just the previous version, it should be ok.  but if he is holding the previous +++ version, he may need to prepare more doc.

if he is entering hk with canadian passport, they will only allow him to stay in hk for certain days(just like other visitors), you friend will need to get the new hkid before the expiry date, otherwise he may get caught.