What is the ruling on using a prohibited discount card that the husband bought before knowing of its prohibition, and is it permissible to benefit from it until its expiration date?

The preponderant opinion is that dealing with paid discount cards is impermissible (haram) due to the presence of gharar (excessive uncertainty/ambiguity). The purchaser pays money without knowing what they will receive in return. The loss is certain, while the gain is merely probable. Therefore, it is not permissible to buy or deal with them. If it is possible to annul the contract and get your money back, then do so. Otherwise, you may use them for the remaining period to recoup what was paid, then refrain from using them in the future.