Petunia's problem with magic always seemed to stem from jealousy that she wasn't 'special' like her sister, so I'd say Dudley being a wizard might put him and Harry on much more even ground, actually. As their son or not, Petunia and Vernon wanted 'normal'. I'd imagine they'd be just as keen to try squashing the magic out of Dudley as they were with Harry, and treat them very much the same.