That information is in a wsm, but without it you and he are shooting in the dark as to how far you can safely go.
This problem is either the ECU is fried, OR it's not getting the right inputs. (Or both!).
There's nothing unsafe about taking measurements. There's also nothing unsafe about providing ground signals downstream of the ECU to verify those components. If it were jumping 12v in there, yes, there is a risk with that, but there is also a way to eliminate the risk (disconnect the ECU from that terminal). But simply grounding a relay terminal that is connected to an ECU that would ground that same terminal is not going to bother the ECU at all. So, there's nothing unsafe here. He's not going to fry anything. But, we're past that stage anyway, since we know the ECU is not grounding that terminal. That leaves verifying the inputs.
He can safely troubleshoot the inputs, and there is unlikely to be anything there that he can't fix himself. So, if he finds a problem with an input (once he knows what they are) and it's something simple like a bad switch or sensor, it's a big win.
If the inputs all check out correctly, we'll know it's the ECU, and it's dealer time. I'm sure at that point he'll find a way to cross that bridge.