If you need to call in a witness, the witness should be another member of management, not a peer or coworker. After all, management has an obligation of confidentiality regarding delicate matters like progressive discipline. Peers, on the other hand,may share inappropriate information with others and consequently breach privacy.

It’s recommended that you hand-write the following on the signature page of the warning:
The employee refused to sign. Notified employee that these are our findings of what occurred and that he would be expected to meet these standards of performance/conduct.

At that point, the manager should sign the document, and the witness
should sign the document as well. The witness may then leave the room. Remember, the witness isn’t there to be brought into the situation or otherwise pass judgment. However, the witness mustbe in the room with the employee present and must also receive verbal instruction to sign the warning in the employee’s place. Witness acknowledgment is then complete. You should give a copy of the signed document to the employee and keep the original for your own records.

Taken From : The Hiring and Firing Quention and Answer Book