"He did not gift anything to anybody before his death."

Yes, he did. You need to look at Timing.

"Client finds out he owns property next door to Mr Jones after Mr Jones dies. Is it a gift?"

It was Gifted in 2019, according to what you are telling us.

"He just did the quick deed on this property."

Yes, that is how he gifted Value to your client.

"No clue what to use for FMV on this sale."

You had no sale, in 2019. There is no applicability of FMV.

"Joint w/ right of survivorship? Yes"

Then your client inherited 1/2 at the FMV, and it seems the client's sale is timely to the DOD, so use that sale price for FMV.

"My client had ZERO to do with the estate or sale of any other assets."

You keep stating things on both sides of the fence, and either they both don't apply, or they both do apply. That is confusing us.

"Who knows what Mr Jones meant it to be."

It is either Sale or Gift, in 2019.

"Is there issue with gift tax in this situation? Now it's not just a BIG shock, he has to pay gift tax on it because Jones neglected to do so?"

Gift tax is the responsibility of the Giver.

You've brought a lot to the topic that does not apply. You need to narrow down to what applies and drop all this other stuff. There is a lot of protesting going on, for who didn't know what, and that doesn't matter. What IS and what legally exists, is what matters.

The probate and estate documents will be helpful. The legal papers matter. What someone else remembers or thinks or intended, does not matter.