You definitely need to contact Canada and get his DUI cleared up before you head north. After 10 years he probably will be fine, but why risk it and have everyone nervous before you even reach the border?
Worst thing that could happen is he is refused entrance to Canada or you are delayed at the border as you explain everything. Best to look into this now and have it resolved. It is very likely that after 10 years that they will let your friend in but why risk it and your trip.
Clear it up, have the paperwork with you and remove the stress BEFORE you get to the border.
Here is a good website that walks you through some of the FAQ regarding getting into Canada after a DUI.
DUI FAQ