Assuming he's not getting his gas supply from his own private pipe in the ground, it's already being metered by his supplier. I would recommend a flashlight, and finding out where the meter is.
If he is in a shared premises, with multiple customers being supplied by one meter, and wants to split the bill amicably with them, it's a trickier problem. Each user could calibrate their appliance usage by using gas for (say) 5 minutes and reading the meter, and then keeping track of usage times. It's open to abuse, and not very accurate, but may be better than nothing.
Local government tends to regulate the heck out of the gas supply, with good reason, as any faults can blow up buildings and kill people. Breaking a pipe and installing your own fully qualified gas meter will be a job for the regulated professionals. Breaking a pipe and installing your own home-made gas meter will simply not be permitted. Building your own meter would be tricky. As the pressure drop must be very low, and the flow rate is very low, you need a mechanical positive displacement type device, not simply a fan in the gas stream.