What kind of martial Art is your friend interested in? Do you have an idea where in Montreal he/she will be living? it's a fair sized city, no sense recommending a place that is on the West island to someone who will be staying on the South Shore or East end.
Check out GAMMA, original Dog brother, kajukenbo, muay thai, BJJ, FMA in downtown Montreal, or Senshido with Rich Dimitri. lots of Mas Oyama style karate in the area, several options for BJJ or MMA training.