If one is requested to be onboard as an instructor in the interest of safety, and acts mainly as pilot monitoring but also flies the plane and assists the operation, is that not legally acting as an instructor?

Keyword: safety. And the owner wanted a CFI onboard.

If the CFI wasn’t there perhaps the pilot would have killed himself trying to do the operation solo.

Also, we are hypothetically talking 100 hours total here. Not hundreds.

It's obviously a grey area, as you can see here. My one case study was a more extreme example, but may be worth considering. You have to make the call.

If you could find an FAA legal opinion that JB mentioned, that might help.

If you’re going to be onboard as an instructor, then be paid as an instructor. It is that simple. I’d say to stop whoring yourself out, but even hookers get paid.

Falls under an unpaid Internship in broadcasting journalism with an emphasis in aerial traffic reporting. Make sure you get a nice letter of reference for completing your internship from your mentor.

That income can be expensed out fairly easily in the grand scheme of things if it is just temp work, IMO.

Now if you are planning to do this a lot over a long period of time, when/if you get audited, the IRS will probably determine that you should have paid taxes and then it will be up to you, and your lawyers, to prove them wrong or reach a settlement.

BTW, you should also ask the owner how he is accounting for it. He may be trying to expense out his plane at a loss so his hobby is being paid for as a business tax deductions. If he gets nailed by the IRS, you will get pulled into an audit as well.