The law says you must yield to the right if you're being overtaken. So, yes, what the relative did was illegal
In other words, driving in the passing lane is not illegal until somebody is coming up behind you. But, common decency would suggest you use it to pass, then move over.
(3) On a highway having more than one lane in the same direction, the operator of a vehicle traveling in the left general purpose lane:
(a) shall, upon being overtaken by another vehicle in the same lane, yield to the overtaking vehicle by moving safely to a lane to the right;