Since he is a Canadian national, I do not think he should face any problems in exiting Qatar through another border or getting a visa at the Saudi-Qatar border on his return. He may need to get a new visa issued at the Saudi-Qatar border as his previous visa would have been utilized once he exits Qatar. However, just to be sure, I suggest you pay a visit to the immigration office and find out for yourself. It is advisable you go there with someone who speaks Arabic fluently.