You need to involve the sales head since the manager concerned is part of his department. You can either meet up with the sales head and tell him the facts and the need to do a one on one or you could tell the sales head to join you in the meeting with the concerned sales person. If I were you, I will tell the sales head to join me in the meeting. This will help in ensuring his buy in, for any decision that needs to be taken post the meeting, since he is part of the deliberations.