1997 BMW 840 transmission will not Move the car in gear no

Mechanic friend drained the oil in my transmission and replaced the transmission filter. After the fact he did not know how to replace the transmission fluid. I found out later that you never have to change the fluid in this car's transmission! My fault! I had the car towed to BMW here at home and they charged me actually the bill is already $850.00 so far and I have not picked up the car as they claim they did a bunch of tests and cannot find out what is wrong with the car and to add insult to injury they are the BMW dealer here in Las Vegas if they cannot fix the problem who can? So I would ask you what can I do to get my car running again? The car ran perfect before my big mistake but I thought all cars with that amount of mileage should have the fluid and filter changed! It is a great car and has been good to me and I really love the car and want it back on the road running just as well as before. I feel terrible now as the car is such a beautiful car! Can you help me? I will make a donation to your technician as soon as I get a response and thank you!