Actually he should be able to use that gas tank. Those sender units work on ohms not volts.
What you need to do is find the ohm readings on the sender for full and empty. Then you need to find the corresponding readings for the gauge. Then you need an adapter to convert between the two. That "reducer" may already do that conversion for you.
If there are two wires on the sending unit, one goes to the gauge, and the other mostly likely goes to ground. I have no idea which is which.
