There’s a few ways to accomplish this. He could rewire the connectors on the 2x12 to do series, but it’d take a little tricky work with switching jacks to allow the cab to be used without the second one attached. Plus, you could never use the two cabs in parallel (if needed) without rewiring the jacks. He could also rewire his outputs inside the amp, but it has similar limitations. The other option is to get a y-cable (not sure if these even exist) or a box to perform the wiring between amp and cabs.
For fun, I'm making him a little connector box to use. I had planned to use a (metal) utility box I had on hand for this, but realized when I was looking up the wiring that you can’t use a conductive box between the two jacks for the cabs, as it will cause a direct short across one of the speakers, which means you'll be back to half the expected load. Definitely not the best thing to do
