Speak to the priest or deacon directly about their financial difficulty in paying those amounts–a church secretary or other staff will often not be able to make a decision about these things, and possibly may not even know if there are other options, and if the priest or deacon is not aware of the situation, he is probably going to assume that it’s not a problem. Generally (in the local Catholic parishes here, at least), what may be called “fees” are really suggested donations, not a set price. Most people try to make a donation of the suggested amount as they are able; some give more, some less.
That said, we do have some 18 weeks before Easter (of course it depends on when exactly the marriage validation/blessing would take place), so would they be able to set aside a little money each week to make a donation that is similar to that amount? And if you had planned to give them a gift for their validation ceremony (and/or her a gift upon entering the Church), perhaps you could help out with the fee/donation instead?
Just my thoughts.