Has she sought a dispensation from form? She needs to apply to her Priest for this, without it, she will be entering an invalid marriage, committing adultery, you would not want to attend such an invalid wedding.
If she has the dispensation, as long as they are both free to marry go to the wedding and pray hard - statistics say your granddaughter’s children will not practice the Catholic faith, children usually follow the faith of the father. Big prayers that your grand son in law will convert to the Catholic Church! Pray and fast for them.