Can my friend enter Canada with a criminal record?
Chances are, no. He will have to produce ID to enter Canada, normally a passport, but, that is not legally required for Americans crossing by land, but, some form of ID will be needed. They will check with the computer data, and any criminal record will pop up. IF the offence was also an offence under Canadian law, and was a long time ago, and it was not to serious, and there are no other offences, he MAY be "deemed rehabilitated," and allowed entry. But, in general, criminals cannot cross in to Canada.
A standard query asks if the listener has ever been arrested. Failure to honestly answer has immediate consequences. Thanks to on-line services Canadians know most of the answers and just want to know if my reader in inclined to be dishonest. Stay factual and do not go off-topic.
Depends on what the record is for as well as how long ago it happened. Yes, border guards can ask and they have a lot of information available to them with computers.
MAYBE Canadian Official can access and do access US Police data bases.when they check your Identification at the Border. It will depend what his bad deed was under CANADA'S laws and how long has passed since he completed the penalty. https://www.canada.ca/en/immigration-refugees-citizenship/services/immigrate-canada/inadmissibility/reasons.html
As soon as your friend's ID is scanned. Records are shared.