I faced virtually the same dilemma a few weeks back. Fortunately it was a weekday morning. The City called the owners # on the water bill, and a friend of the owner's (who were both at work 1+ hours away) came out and turned the water off. That evening they had the pipe break repaired.
I was going to turn the water off if it continued to leak after 5pm.
Was it the right thing to do; morally yes. Legally, probably not but your "exposure" would be minimal.