If you're asking whether a "[l]awyer in the US" (which I assume means one that lives and practices in the U.S.) can represent a client in a Canadian court, the answer no, unless that lawyer is also admitted to practice in the applicable Canadian jurisdiction. I feel fairly confident that there are some lawyers who live in states that border Canada who are also admitted to practice in Canadian jurisdictions.
sungalaxy has another thread with the same legal question. It was locked because this really is not a US law question. sungalaxy needs a lawyer in Canada although s/he was advised to check out international law firms in Australia.