However, it might be possible to have the divorce set aside. The couple would want to file a joint motion to set aside the divorce decree in the same court that granted the divorce in the first place. This should be done quickly while the court still has what is called plenary power. This is typically within 30 days from the time the judgment was filed. If the couple is successful, this will put them in the same position they were in before the divorce (as if the divorce never happened).
Alternatively, the couple could always remarry. There is normally a waiting period after one is divorced before that person may remarry, but that waiting period wouldn't apply under these circumstances. Probably the most important practical distinction between this option and the option above of setting aside the divorce is that remarrying would not reconstitute the previous community estate. The community estate from the prior marriage (that was divided by the prior divorce) would remain separate property, and a new community estate would be created.
This situation would be complicated if children, child support , custody, and visitation were involved in the couple's prior divorce (if the couple could not have the divorce set aside). Depending on the particular facts, modifications, premarital or marital property agreements, and other options should be considered and discussed.