Canada is very strict about DUI's and entering if if there's on on your record.
Re the travel insurance - it's an interesting question, most insurers will not pay if you are denied entry due to not having a visa or passport. But what about a situation like this where you may not have an idea that you will be denied entry? Can't hurt to file a claim but if there is anything salvageable on the ticket or other expenses that would offset whatever he'd be able to recover.