For a car as expensive as this, towing and paying someone to drain the tank doesn't seem so bad.
However, there are other options that I've used on older/cheaper vehicles in the past.
1. Buy a fuel additive/stabilizer. This is supposed to be added before the fuel sits, but I seem to remember one saying it could be added to old gas.
2. Siphon what you can, then fill the tank right up with fresh fuel
With either method, rock the car a bit to ensure good mixture

What did the BMW dealership recommend? In theory, a plug-in hybrid would/could typically live a long time without using any fuel...