That's not the full description of what happened:
It's also not correct to say, "we don't do prams" - it's that, "we don't do prams for passengers who aren't disabled, elderly, or less mobile". They could've taken 10 seconds to explain that passenger assist is for those groups, not just any passengers who have prams or heavy luggage. Given the Passenger Assist forms I've just sampled, most of them don't actually require selecting a disability, mobility need, etc; you can indeed tick that you need help with luggage and proceed to booking. It doesn't sound like RPM's pal intentionally booked it knowing it wasn't designed for them.
I mean it's been roundly pointed out to RPM in this thread that this isn't the intended use of Passenger Assist.
As my mum would say though, "two wrongs don't make a right." Passenger mistakes are common; unless there's reason to believe it was an intentional misuse of the system, there's no excuse for poor customer service and loudly complaining about passengers as you walk away from them.