They should apply now in family 2B. The petition can later be upgraded to 1st preference when they get citizenship, while still retaining the original priority date. They've already wasted 4 years by not applying for you in 2007.
But you have to make sure you don't get married before the petitioning parent becomes a citizen, as that marriage would void the petition. If you marry after they have citizenship, you would only downgrade to 3rd preference, which would add some years to your wait but your spouse would be able to join you.
