I planned on having my girlfriend urinate in a cup before she leaves to work and keep it in my car daily so when the time comes I just put it in the whizzinator bladder and strap it on and good to go.
Keeping your GF's sub pee in your car daily will not be a good idea. Human urine needs to be refrigerated or frozen until ready for use. For your particular situation, consider a synthetic sub since it can be stored at room temp. Just be sure to keep the synthetic sub away from direct sunlight.

Human urine is only good for 3-4 days in the fridge, and up to 1 year in the freezer.