Okay, it depends on the roaming agreement with his provider and either Rogers, Telus or Bell. Freedom might be included in this and VideoTron (Quebec). I live in Canada.
LTE should work fine, but don't be surprised if one of those providers kick you onto the 3G network. Providers put network subscribers first, Canadian Roaming next, and then everyone else after. It's different for me, since my provider (Freedom) has roaming agreements with Rogers, Telus, Bell and AT&T to use their LTE network for data.
As for buying an iPhone in Canada to use it in Europe, I strenuously suggest you buy it in Europe. As has been documented here, iPhones do not have an international warranty and should you develop a problem with your phone you have to fly back to Canada to avail yourself on the warranty. In addition it may have issues connecting to 4G or an LTE network (the iOS 11 manual treats them as separate, so in this post, they are treated as separate).
Hope this answers your question.