That's not true:
Hot water being used for her showers, hot water being used when she's washing her dishes, electricity and gas being used when she is cooking, lights being used when she's in the bathroom/bedroom, electricity being used when she is charging her phone/laptop, electricity being used to wash her clothes/towel, the boyfriend putting on the heater because she's cold....
These are all little things, but effectively these are times where it's costing for her to be there, and she's not paying for any of it.
I've been in this situation before. My housemates girlfriend was having up to three showers a day at our house and not contributing at all. I told the housemate she had to contribute or stop having showers all the time, they moved out