Do you have any experience with heavy flows..? I change my pads every hour some days and I never ended up standing in red water. A ten minute shower uses up 75+ litres of water. She's only going to produce a couple of drops of blood during that time, no matter how heavy her flow is. Look at the size of a menstrual cup - even for women with heavy flows, that usually lasts hours and it's still far smaller than the water a shower will take.

If your girlfriend is actually bleeding so much the water is constantly red, that sounds more like a huge wound in her vagina/uterus that's bleeding out. Like if she's bleeding that much she's probably dead at this point. Rest in peace?