This question is often raised when a U.S. citizen is trying to travel to Canada. Whenever a citizen of the US travels to Canada they must go through customs and declare if they have any prior convictions. At that point it is up to the discretion of the customs officer wether or not to allow the individual into Canada.
Your question is the exact opposite in that you are asking if a Canadian can come to the United States.
There are no areas on U.S. declaration forms to list if a non resident has prior convictions in their country of residence. So yes the individual would be able to travel to the United States.
A link to a sample declarations form is as follows: p://www.cbp.gov/xp/cgov/travel/vacation/sample_declaration_form.xml
As far as a pardon goes, you are going to have to speak to an attorney in Canada.
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