I have literally walked into a gym shower and seen s*** all over one of the stalls and the floor. So, yeah I always bring flip flops if I'm planning on using the showers.
12 lbs of water weight is a lot. Assuming that you're achiving your target HR during the cardio sessions are you using the same scale? Is it correctly calibrated?
Realistically you can't expect her to change; some people are fat their entire lives. You said yourself that she didn't bother to workout when you began losing weight.