Does you dad have any actual paperwork from the divorce (there must be something he signed to make the divorce final). I have no experience with divorce decrees, but i'd imagine that they state the original date of marriage.
If the place your dad needs to get a copy of the marriage certificate really can't come up with a copy, I would strongly ask them to write a letter of non-availability stating the reason why they can't produce one. (whatever the reason, it is their fault if they don't have their administration in order)
If you can get these two things, and provide a cover letter explaining, you might be able to get by.