The 5th wheel pin will mate to the commercial tractor's 5th wheel - the pin is identical to the commercial version. The height may be a little off, but as long as you aren't in danger of dragging the trailer's landing gear it should work. Be careful of the 5th wheel placement - if it's a slider your brother will want to move it all the way to the rear to avoid having the back of the tractor hit the front of the trailer.
How long is the tow? One problem is you can get metal fatigue in the 5th wheel's frame on a lengthy tow. Even if it's air sprung tractor, the 5th wheel saddle on a commercial tractor rides rougher than one in a pickup truck and the tractor is a lot heavier, so bumps and jolts send a lot more energy into the trailer frame. It's not an issue for a short, slow tow, but can be a problem on a long trip. People who convert Class 7 tractors to tow 5th wheels almost always replace the commercial 5th wheel platform with an air ride hitch because of this.
The lights use a different connector - the standard RV connector has 7 flat pins while the commercial truck connector has 6 round pins. Either replace the trailer connector or make an adapter.
The 7th pin is for the electric brakes. You'll need an electronic controller like you'll put into your pickup truck to activate them - they won't mate to the tractor's air brakes.
You can take the 7th pin, the electric brake line, into the cab and temporarily mount a controller for the electric brakes. Or go without trailer brakes. It's not exactly kosher but the tractor brakes by themselves should be able to stop the combination reasonably well.