A friend of mine is considering adding a gas heater to his outdoor pool.
Without getting into too many parameters (size, location, usage etc) I
just want a few price points to get an idea of the magnitude of the cost
others are paying for energy (not the equipment).

To those who own heated pools:
If you would, please estimate how much more your energy bill is per month
to heat your pool and tell me what region you live in and if you heat it
full time or periodically so I can get a feel for the cost.

I assume he will also add an insulated pool cover at the same time if it
even gets done.

He is not a candidate for passive solar as he would like the option of
heating it in the winter if he wants (mild SF bay area winter)

The answer is a definite YES!, NO!, AND MAYBE!

Pool heat characteristics can be affected by a lot of things.

Surface area in relationship to depth.
When you run the filter.
Night time temperature.
Wind.
Air temperature.
Amount of sunshine into the pool.
Whether or not a blanket is put over the pool during night time or in windy
conditions.

A pool can vary ten degrees a day considering a combination of these
factors.

So, to answer your question cost ............. A LOT! Gas prices
(whether propane or natural gas) is high. The heater would have to run a
lot during the winter to maintain a constant temperature. It would have to
be turned on hours in advance, depending on the heat gain rate of the pool.

As an example, my spa heats at 5 degrees an hour, and it is only 350
gallons. My pool holds 100 times that much water, so I can only guess how
many degrees per hour it would heat up, or how many hours per degree,
whichever is better.

Heating a pool is a losing deal. The heat will be lost to anything that is
cooler than the water.

How much does your friend want to pay? The only way to really find out is
to hook up the heater and go for it.

I use a solar heater, and that works great. It gives us a few extra months
of warm water, and during the summer, we have to turn it down, or the water
is not enjoyable to swim in. Don't discount solar, as it is free once it is
installed. The cost offset will be substantial, as a solar heater will cost
about $3k, and then after that there is no cost. If you burn $3k worth of
gas, you got nothing to show for it.

I live in Las Vegas.

HTH

Steve