If a pilot is a holder of both Canadian and FAA commercial pilot certificates and is flying regularly in Canada, does this pilot still require a Flight Review to maintain currency of their FAA certificate?
FAR part 61.56 ( [U.S.]Flight Review ) requires a U.S.flight review (or authorized alternative - see below), for anyone operating an "N" numbered airplane within the U.S. (when using a U.S. certificate/or U.S. certificate based on a foreign certificate), even if that person is current in another country for which he/she also holds a certificate.
See also, FAR 61.75 (Private pilot certificate issued on the basis of a foreign pilot license.)
See also this FAA Legal Interpretation which relates to your question: need for Flight Review for holder of U.S. cert based on foreign cert in accordance with FAR 61.75
NOTE: In accordance with FAR 61.56 there are various types of pilot proficiency checks or practical tests, etc. that may be substituted for a flight review (i.e. see FAR 61.56(d)-(f))