It looks like we had our wires crossed.
That's not what I was saying he can't do. There is obviously no reason why he can't measure any parts of the other circuits with the 'scope. I have never suggested that there was.
What I have being trying to say all along, is that he should be careful about where he places the current meter's probes, not the 'scope's input which will be fine as long as he doesn't connect the 0V to anything but 0V.
Maybe I didn't make this clear enough?