I think that there is vigorous agreement all the way around here - a "flight review" is not instruction. It's a great idea, and I encourage pilots to fly with peers and instructors from time to time to refresh their skills and confidence. From the bad experience of one colleague many years ago, I have one piece of advice: As the "reviewer" is not intended to be PIC, actually check that the intended PIC has a valid license and medical, and that the airplane is properly insured for the flying such that you do not become liable for it. As to whether payment for service can be made/taken... an other aspect to be certain about - TC might view paid reviewing as a commercial activity.
My friend, as a favour, checks out new pilot on the Grumman Tiger he just bought. I had delivered it to the aerodrome from which they took off for the training (which is how I was drawn into the situation). They fly for a few hours, then run it out of fuel (okay, partly on my friend, he should have been paying much more attention to that!). I got the urgent call: It's upside down off the end of the runway, how much gas had I left in it? - Full tanks, less the half hour delivery flight - lots. 'Turns out that new owner pilot misrepresented himself to my friend - medical not valid - so he was not PIC, my friend was. Suddenly, my friend is responsible for an accident and the value of an airplane, whose insurance did not name him as an insured pilot. It was messy, and my friend was just trying to do a favour, 'didn't even take payment. If you're right seat, and don't think you're PIC, may certain that the other pilot is.
And, from personal experience: I'm giving a fellow his float rating in an amphibian. He's flying from left seat as PIC (he'd had the plane of over a year, flying it wheels only, flew it very well), we depart a runway. We land on the water, and I fail to correct his error - the airplane, and we get wrecked. The airplane had no hull insurance, that was agreed before the training began, and a hold harmless was signed to me. What I did not think about was that although there was valid passenger insurance on the airplane, I was not entitled to make a claim. I became the PIC as the keel touched the water (he did not have the float rating yet), so as PIC, I was not insured as a passenger - no injury money for you!
So just know what you're signing up for when you offer a much needed service to other pilots!