I owe my neighbor a favor - he has been kind enough to plow my driveway several times over the years. The other day he dropped his bike off, and asked that I replace the cassette and chain. His bike is all Shimano, including the shifters & derailleur. He had purchased a SRAM chain and a Campy cassette. All are 10 speed components. How interchangeable are these components from 3 different manufacturers? I don’t want to spend 1/2 a day trying to adjust the shifting.

Chain will work. 10 speed is 10 speed.
Cassette has a totally different spline pattern than SRAM/Shimano.

If the current cassette is Shimano, the replacement Campy cassette will not fit onto the hub.

The Campy cassette is the odd-ball here. If the hubs are Shimano (or Shimano compatible), then the Campy cassette wont work. Otherwise, a 10 speed drivetrain is compatible. For example, right now, I run Shimano Ultegra cranks, SRAM 1070 cassette and KMC X10SL chain. I've also used SRAM chain Shimano chains and Shimano cassettes at various times.

Even if he has a Campy freehub, the 10 speed Campy spacing is different than everything else and isn't really cross compatible.

10 speed Shimano and SRAM chains are cross compatible, although they may or may not work slightly better when the chain and cassette are appropriately matched (and I generally try to do so when possible).