It's quite possible that the driver might trigger a fault. Or it's possible that he was referring to the inductive spikes of switching an active circuit.
It sounds to me like you're trying to save the $90 or whatever of buying a second controller, by instead dropping in $20 of relays and relay control circuitry. So, your current project plan will be:
- $90 first controller
- $20 relays
- total: $110
What will very likely happen, is that the project doesn't work like you intend it. Let's say that the problem is inductive spiking, and you fry the controller. You then learn that the intended design doesn't work, and you have to buy two controllers to make your project work. The end budget of your project is then:
- $90 first controller (dead)
- $20 relays (not used)
- $180 two more controllers (working)
- sum $290
An alternative is that the project doesn't work because of open circuit detection, and the first controller doesn't die. In that case, the sum of the project is:
- $90 first controller
- $20 relays (not used)
- $90 second controller
- sum $200
If you want to aim for the $110, knowing that it may very well not work, and you may end up paying $290 to get to the end goal in the end, then that's probably a fine learning experience.
If you just want your project to work, I'd highly recommend this option:
- $180 two controllers (works)
Also, if this will control equipment with spinning tools or heavy construction, where people are nearby, the safety factors are important. If someone ends up with a blind eye or crushed hand, $70 in savings isn't really worth it. If this project doesn't get close to people or is very light duty, then that's perhaps less of a concern.