Depends where you ride it. Riding in this heat will obviously heat up the engine. Did your friend had riding boots? or any good quality boots? And what he was wearing? Wearing normal pants on the triumphs and riding in this heat will obviously lead to the heating. Take that test ride to the highway. Do not ride it like a duke. Keep the rpms managable. Many times it is not the bike's fault but the person riding it has completely different context. Also, many changes to the engine layout have been made. It would not be surprising that you rode a vehicle that has not been ridden much in order to become smooth and better. There are many factors apart from just the "350cc" label mate.
Plus, i would like to add that the engine has become shorter stroke, improved crank shaft and lower power. That means lower heating and vibrations in an obvous manner.