I assume your wife has a multiple entry visa for the U.S. and one for Canada if applicable. Could you check that? You don't want her going to Canada and not being able to reenter the U.S. in order to return home. Also, she might want to ask an U.S. immigration/BCIS supervisor if she has to turn in her I-94 for short visits to Canada (or Mexico). It's not required for those entering under VWP.

Anyway if she has to turn her I-94 in, she hands it to the gate agents on her flight to Canada.

On here return, she does indeed preclear U.S. entry procedures at YVR and her arrival into the U.S. is considered domestic for all intents and purposes.