Dispensations are to be given before the wedding, not after there has been a divorce and the parties are trying to get an annulment. At this point it is too late to ask for a dispensation. If there is an impediment that is discovered after there has been a divorce then that would be evidence that the marriage was null and void to begin with.
Yes, impediments are supposed to be known prior to the wedding, but that does not always happen.
The OP made no mention of the couple being divorced. But even if that were the case and there were impediments discovered after divorce, the marriage is still valid and dispensations can still be granted - in order to preserve the marriage.
Per what was said in the OP, the marriage is presumed valid. If after investigation by the marriage tribunal they find some impediment, they can and if possible will grant a dispensation for that impediment. Even some diriment impediments discovered after marriage can be dispensed with, some cannot. But the Church always goes to great lengths to preserve the marriage, not takes sides in pity for one or both of the spouses.
Seems like from my armchair, what it amounts to in this case, is that they married for the wrong reason, but they married.