If the owner wanted your parents to leave before the end of the lease, they should have been given official written notice, otherwise they were under no obligation to go anywhere. Now, because the owner never actually provided anything in writing to the effect that he/she wanted them to vacate the property, it is being interpreted as your parents just wanting to break the lease with no reason. A grey area for sure - but the RE are not necessarily in the wrong for asking for the extra rent.