This is a fairly classic ad hominem comment.
What difference does it make who makes this argument, if the argument itself is sound? On the other hand, if the man is 50% saint, 50% angel, and 50% genius, what difference would that make, if the argument itself was terrible?
Comments suggesting that he shouldn't make this argument do not actually address the argument itself. In the best case, they are a distraction from the subject being discussed.
In the worst case, they are a form of tone policing, which is a tactic for silencing sound arguments by complaining that they are not being made by the right people, or in the right way. Example:
A man murders another man. The police torture him into a confession. He complains about the torture. Is he the last person that should be complaining about violence? But if not him, who? Must we wait for the perfect angel of an innocent victim of police brutality, before we act on it?