It is good policy that if an employee (who is being reprimanded) requests a witness, one should be made available. Likewise, if the employee wants a witness the manager may want one too.
I can only address this question from the perspective of conflict management. You may want to also get a legal opinion from an attorney in order to address this question and the legal ramifications of this scenario.
Elinor Robin, "The Relationship Mediator," has more than 18 years of experience in mediation while working within the public and private sectors.