Hi Udella,
Permanent residents who received their Canadian perm. residency prior to June 2002 were issued an IM 1000 form, basically it was a legal paper sized form that proved their permanent residency and was stapled in to their passports.
As of December 31st, 2003 all Canadian PR's need to have the Canadian Permanent Residency Card (a small plastic card - looks like a drivers license) to travel across the border on a commercial bus, train etc etc. I don't know if your friend has applied for this or not, however they should apply as soon as possible.
They can apply for the forms here:
Application for Canadian PR CardThe card currently costs $50 and is good for 5 years (previously the IM 1000 had no expiry date).
Even though they probably weren't on a commercial transport and even though he was probably just sent through with a provincial drivers license etc before, technically you have to prove your Canadian citizenship (or in his case PR) when you cross the border by land.
What he was probably given was an I-94 Arrival/Departure record. It is really important to turn it in, they do keep a record and when he tries to cross the border next time he will probably be grilled about it and they may not let him in (this happened to my Husband once, not pleasant - luckily he had a stamp in his passport from another country that superseded the I-94 date).
He can mail it back, information on this can be found here:
Forgot to turn in your I-94 or I-94W? He might want to make a photo copy and send it express post with signature, to ensure no hassle in the future.