A colleague told me a story the other day. He advised a reporting staff member about how to handle a routine situation that the staff person had not encountered before.
Within minutes, a peer of the manager told his staff person that the advice was questionable and that the staff person should check with the manager's boss before carrying out the advice. The manager asked me what to do about this situation of the peer manager redirecting his staff person.
I suggested that, with a peer like her, he has to set limits. If he doesnt set limits, she will continue to trespass into his relationship with his reporting staff. (More appropriately, if she disagreed with his advice, she should have sought him out to discuss the advice not his reporting staff person.)
To set limits, he needs to go to her and tell her how she violated his space. Then, the two of them need to agree about how such situations will be handled in the future. If he doesnt make her clear about his limits, history will repeat itself again and again and again.