TBN is a good idea if you are going to do a UOA, although I would save the money and just change the fluid out.
Look at the colour of the fluid and smell it. If it looks darkened and smells like burn't steel/plastic/rubber then it's probably used up.
If it's a nice red colour and smells sweet or like virgin T.F. then you are still good to go.
By no means is this scientific but it has served me well. I'm at the point now that I just change out my fluid at a regular interval regardless if it needs it. My transmission operates silky smooth!
Transmission primarily suffer from heat which degrades the fluid, the fluid doesn't suffer from the same sort of abuse as engine oil in an engine because in a transmission there is no combustion byproducts leaching into the oil.
The TBN will tell him how much of the Add-Pack is remaining. That's good information considering an ATF's job is to keep the system lubricated and clean. Heat can seriously destroy the fluid. Now getting a TAN, would be a waste IMO!
TAN, but you'll need a VOA for comparison.
Save the TBN for engine oil.
Excessive wear and shearing are things to look at. 25% loss in visc is a reason to flush the atf. Wear is transmission dependent. 100ppm per 30k or less is a goal.
Insolubles and those wear particles, will chew into all your seals, clutches, and help carve into soft metals.
Antifreeze means your built into the radiator ATF cooler is leaking. Excessive water could mean you need to spend less time offroading/snorkeling thru the local river.
What it comes down to is your transmission type, driving style, weather patterns, fluid used, and any 'mods' to the filtering, cooling, shift patterns, engine power......