Mohamed faces scrutiny over his "coaching" only because Nick undergone a miraculous transformation. Funny to imagine that if Nick didn't wake up and lost (as he should) then there would be no problem with Mohamed's umpiring. But I'm not surprised at the outcome because Nick is so unpredictable in everything he does.
I think Mohamed's intention was to say something like: "Listen Nick, it's clear to me you're tanking here. If you don't stop it and do not engage in the match per ATP rules, I'll have to give you a warning". And he decided to leave his chair because the crowd was too loud and Nick, in his amok state, would not hear him. If that was what Mo said, it would hardly be coaching but rather umpiring "pre-warning". But in this case, did Mo make a mistake by delivering his "pre-warning" rather than simply a formal warning without any talk? I think he did. Even assuming that he did not want to favour Nick against umpiring rules. By delivering his pep talk, Mo has showed that he would feel uncomfortable with a possible explosive reaction by Nick to the alternative formal warning. In other words, Mo felt afraid of Nick or felt passionate for Nick, which boils down to the same mistake really: impartial umpire must not care about the feelings toward players while on duty. Of course Mo realises his mistake & he will emerge OK from the scrutiny because otherwise he's a good ump. They scold him for his mistake and he'll swear not to repeat it.. As Fed quipped during his press when asked his opinion on Mo's umpiring: "we won't see it again".