If I couldn’t trust traffic to have a backbone and admit to what they have told me, I would record all conversations, have my own dashcam and put the phone on load speaker and save…
Also if the final warning isn’t in writing then it won’t stand up at a tribunal, there are also procedures to follow before issuing a final warning.
If its a verbal or a threat, just say constructive Dismissal. Then see the ■■■■ squirm!!