presumably all the same size? How worn are the Dunlops? You could look up the specs on both and see how far off they are, comparable tread patterns. Is there a reason he wants to do that other than thats what he has on the shelf or can get easily? Id prefer to match the tire if it were me but if the tire dimentions are comparable and the tread is consistent It would probably be ok. Keep in mind that you may find the three Dunlops wear out before the Yoko and then yure faced with the same dilema of buy mismatched tires
my $.02