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.
The TriStar Gym is pretty good. It's a mixed martial arts school so you can practice MMA, Muay Thai, Karate, Krav Maga, Boxing and other disciplines.
The Gelinas Academy of Mixed Martial Arts is also pretty good, but not as big as TriStar and without as many training equipment. But the training is really good and instructors very competent.
Both place are good bets if you don't really know what you want to practice since they offer several training disciplines.
If you need some reviews of those schools, try MuayThaiMontreal.com. They also have some pretty cool Muay Thai videos.