A wider tire will probably hurt the handling of your bike. Don't know for sure but it is probably a 180/55/17 but check the tire you have now. You can probably get a 190 in there if there is enough clearance with the swingarm but trust me keep the 180, going to a 190 will make it handle like shit.
yea, if your friend wants to go with a wider tire, the turning will be very slow and sluggish...lazy to throw around if he ride the canyons.
as what michaelrc51 said, the recommended rear tire size for his bike is most likely a 180/55. if he goes wider, he'll sacrifice handling for the fatter look. he should be able to fit a 190/50 if he wants it that bad though.