Based on my Google and stackoverflow search I'm guessing there are no library for this purpose.
The client wants to reuse parts of an old system (the forms) in which users would download an editable PDF, edit it, and email it back to a secretary. Our users aren't very computer savvy and many don't realize they need to email the forms back, assuming instead the forms somehow get submitted when they save their local changes (or something).
I haven't seen the forms yet, so I cannot assess the viability of an alternative.
At that point you can save them to a database, merge them with a PDF, etc.
BTW - the XFDF/FDF files aren't really necessary to just fill in a form. You would only need them if you want to be able to display a PDF that has form fields and to fill those form fields in automatically. If you are always displaying a blank PDF for them to fill in you can get away with just adding a button which posts back to the server.