Is there a medical explanation for why my girlfriend urinates the bed when she drinks alcohol.?

Yes.: Sleeping under the influence of alcohol is closer to a coma than healthy sleep; loss of control of urine is annoying, but loss of airway control can kill.

How much to drink??: Alcohol has many potential effects that can cause this, but the most common is that she is drinking to a point of intoxication where bladder pressure is not waking her up. Alcohol is also a diuretic and increases urine production, so we have to pee more. If we are too drunk, we don't wake up and can pee the bed. If this is the case, she (and you) are probably drinking to excess. Get some help.