If you right-click the Create IPSW button in SeasonPass you are given a choice of firmware versions.
The problem is that unless you already have the firmware signatures (blobs) saved for the particular version/device combination you are trying to use then you can only restore versions of firmware that Apple are still signing. As of today this is 5.0.2 or later. You do not mention what version you are trying to install.
If your brother-in-laws Apple TV is already on the firmware version you are trying to jailbreak, then tools such as iFaith can extract the relevant firmware signatures. If that is not the case then you are limited to the versions I mention above.