She can ask her grandfather to kick her cousin out of the home, for his permission to break the lock on the bathroom, to forbid her cousin from tampering with the water, etc.
How did the cousin come to be living in the home? If she was renting the entire home, why did she agree to having another tenant move in? Is he also paying rent?
It's unlikely that her cousin has a key to shut off the water service outside of the house, so odds are she can turn it back with the valve at the meter.

