Seems I always have labour related issues...

I have this lady that has been working with the company for about 10 years and with us for about 5 (since we purchased the business) She is a bit of a hard-ass which you need to be in the motor industry, ordering guys around. I've had some complaints of bad attitude before but I attributed it to her no-nonsense approach, and compliments were more forthcoming than complaints. Often it would just be her wording to a customer for example: "Sir, you'll have to wait your turn for wheel alignment" instead of "Sir would you mind waiting a bit for wheel alignment?, I do have a few cars booked before you"

Yesterday things came to a head as I was doing some admin in the office I was called to the floor. I came to the reception counter to find her and a customer in a exchange of word in the realm of "F__k you" and "Jou ma se p__s" I tried to calm the situation, but her and this customer was like fire and petrol. While I was talking to the customer. My sales lady had phoned the police because the customer had threatened to give her "a m__rse klap" At one stage I told her to go into the office, so I can get the customer alone, but in the heat she was out there and back 2 seconds later.

How do I handle this? Do I give her a final written warning for being rude to a customer? Do I charge her at a formal hearing chaired by the RMI for i) Being rude to a customer, and ii) Not being competent for her position, and iii) Insubordination and suggest she either be dismissed or be demoted to admin lady? Do I give her a final written warning and force her to go for training in order to equip her better to handle such situations and communicate better to customers?