More information would be needed to determine if you have claim. For starters, how are you and your father connected to this student loan? Was it your loan and your father was a cosigner? As such, you need to sit down with a lawyer and go over everything. Assuming there is even a claim here, the first question you will be asked is, why did you wait so long (10 years)?
The federal rules dealing with cancellations of student loans are fairly straightforward, but a lawyer is going to need to know the type of loan is involved. The fact that Sallie Mae is collecting it does not tell you much since they are one of the largest collectors of student loans of all types. You will need to bring the letter that you found as well as any other information that you have about the original loan to meet with a lawyer that handles consumer an student loan law. Good luck.