Please consider the practical aspects of addressing a problem before being so dismissive.
Kind of hard to do since this is the first time you've mentioned the camera.
But you didn't NEED to know about the camera. You were already told that the head of the household had the means to determine the required information. Even I didn't
need to know exactly how they were going to do that.
Now that we know this, your precise timestamping of pipe temperature makes more sense.
But, does having this knowledge change the way to approach the problem any differently?
So another idea is a vibration sensor attached to the pipe listening for water flow. This will give a faster response than temperature.
Now THAT is why I presented this challenge here! This is certainly an idea I will investigate.
Thank you!! 
If the security cam has a microphone input, you could even feed this "audio" into it and have everything in one recording.
I appreciate what you are saying, but the idea is to have the data displayed on a chart, so that the usage profile can be seen at a glance, hence a data logger is the way to go. However, I suppose having the audio available as well could be useful - but it's on the opposite side of the building and adding this functionality falls under the dreaded umbrella of "feature creep".