The person to sue is the one who told you that the gas station was legally obligated to accept your VISA card.
Just to clarify...the gas station's contract with VISA does require them to accept his VISA card. So yes, there is a legal (contractual) obligation. The question here is who controlled the denial...the merchant service who processes the card or the gas station chain by virtue of some sort of programming? That, quite frankly is going to depend on the size of the chain.
The most likely scenario is that the link between the merchant service processing the card and the foreign bank/foreign merchant service who issued his card was down...but its not impossible that the gas station chain controlled the denial either.
However, if somehow it really was the gas station chain, then his remedy is not to sue the gas station chain, but to make a rigorous complaint to the card issuer so that VISA would eventually take issue with the gas station chain.