There are a few different methods for accomplishing this:
- Multiple Wi-Fi routers
- VLANs (Virtual Local Area Network) & Gateway routers
He could put each ISP connection on a separate router, and then choose which router to connect to. If he didn't want to do it via Wi-Fi, he could conceivably have two separate wired networks. Then he'd have to have two network cards and plug the two ethernet cables into each one, and choose the network card he'd want to use.
This is the best way to do it. It's more complicated up front, but easiest at the end. This is what we use at the TWiT studios. We use Astaro Gateways. He'll want to search online for how to set this up.