Hmm. In trying to figure out what to do I called the Immigration Canada number and while they said they could give me no information about Buffalo because they can't give information about places outside of Canada they said they would indeed need travel documents to re-enter Canada. Just seems like a big money grab to me...Leon is there any supporting documentation of what you say anywhere on the immigration Canada site?
For example here:
11.2. Verifying permanent resident status
The permanent resident card is the only valid proof of permanent resident status in Canada.
The following documents are satisfactory indicators of permanent residence:
• the original Record of Landing:
• a certified true copy of a Record of Landing document issued by CIC National Headquarters;
• a letter issued by CIC National Headquarters verifying permanent residence;
• a passport duly stamped showing the date on which permanent residence was granted, if the
person was granted permanent resident status before 1973; and
• a Confirmation of Permanent Residence document [IMM 5292B].
...
11.8. Travel document
A31(3) states:
A permanent resident outside Canada who is not in possession of a status document
indicating permanent resident status shall, following an examination, be issued a travel
document if a BSO is satisfied that:
(a) they comply with the residency obligation under A28 [of the Act];
(b) an officer has made the determination referred to in paragraph 28(2)(c) [of the Act]; or
(c) they were physically present in Canada at least once within the 365 days before the
examination and they have made an appeal under 63(4) that has not been finally
determined or the period of making such an appeal has not yet expired.
[Paragraph A28(2)(c) cited in A31(3)(b) above states in part “... that humanitarian and
compassionate considerations... taking into account the best interests of a child directly
affected by the determination, justify the retention of permanent resident status....]
So once they land they will have their COPR which is a "satisfactory indicator of permanent residence" and therefore, they would not need a travel document which is meant for people "who are not in possession of a status document indicating permanent resident status". The airline is another matter as they are responsible for who they transport but if they don't tell the airline that they are PR without PR cards, the airline will see a US passport and let them board.
Also see this question on the PR card FAQ:
Do I require a Permanent Resident Card if I'm returning to Canada by car?
No matter how you travel back to Canada, all permanent residents entering Canada must satisfy the border services officer that they have valid permanent resident status. A valid permanent resident card is the best proof of your status.
If you do not present your card when you return to Canada, you may be asked to show one of the following documents:
a completed Confirmation of Permanent Residence form;
the original Record of Landing (if it is dated 1973 or later);
a passport stamped with the date you were granted permanent residence (if your Record of Landing is dated 1972 or earlier);
a certified true copy of a Record of Landing document issued by CIC National Headquarters; or
a letter issued by CIC National Headquarters attesting to your permanent resident status.
The border services officer may also need to confirm your permanent resident status by asking you questions and checking our information systems.