PCat

If you use your old HKID to entre, You have to change it within a month.  Mine is invalid for few years, when i renew it, I brought my school confirmation letter.  But the officer didn't read it.  He just renew it for me

If you don't want to change to new one, use Canadian Passport, then you don't have to renew the HKID within a month

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.

Littleprince

多謝大家熱烈的回應呀﹐特別是 TOFU﹐十分感謝