I think everyone is giving you really sound advice. If I may expand on the answer, you may want to try Florida Legal Services, found usually in the very front of the phone book. It is a free legal service, and may be a great place to start asking the questions.
The service is typically run by local area attorneys, who are donating the service for community service hours. You will get a great start to finding a solution, and who knows, you may just encounter a great counselor in the process!
"Uncompromising Excellence in Real Estate Service"
Keller Williams South Tampa
(813) 875-3700 ext 288
That said, as a real estate person, the difficulty with non-disclosure on the part of a seller involves sometimes difficult to obtain PROOF that they were aware of the problem. If the roofers that were hired are out of business, who's to know for certain that they didn't lie to the homeowner about having replaced the whole roof? With respect to a warranty, if you know who manufactured the shingles, perhaps you can contact them regarding any deficiency in the product (not installation).
It's a legal question that requires legal advice. If it was intentional non-disclosure, a lawyer will be able to offer possibilities for recourse, which is really the question. You deserve the right answer.