I get the impression that since we don't seem to want to disclose the reason, that it must be somewhat questionable. The 'official' policy on almost all carriers regarding non-refundable tickets is that there is no good reason. They long ago discarded the 'with a doctor's note' policy after it was abused to the point of rediculousness. However, I have found that airlines like Frontier tend to be a little more human when approached with grace and humililty. If you have a legit reason for needing to change a non-refundable, they will usually try to comply.

Also from the OP, I gather that the tickets are for flights already gone [I have two missed flight Frontier tickets]. A very important note here - if the flights were not canceled prior to departure (IE passengers were no-shows), it's likely the tickets have no value at all and the problem is not just the fee. Most, if not all, carriers have put in no-show rules and will invalidate any ticket for flights not canceled prior to departure. So the headache could be much larger....