2017 Land Rover Discovery undriveable after oil change

My 2017 Land Rover discovery sport wasn’t driveable after my husband changed the oil. The gear box fault light came on. It was not able to exceed 15 mph and then it would cut out. I had it towed to the dealership and they said it was low on transmission fluid and it will cost $2241.50

My guess is that hubby drained the transmission instead of the engine. The vehicle probably has way too much oil because he poured extra into a full crankcase.