Is ABS pipe okay to vent a high-efficiency furnace or not?

Both of them are half right, which is why neither would budge. The HVAC guy is correct that plastic venting is completely normal on a high-efficiency furnace, since the exhaust is cool enough that it condenses. The plumber is correct that plain ABS is generally not an approved material for the exhaust side, because the condensate in the flue gas is mildly acidic and slowly attacks the plastic. Over years it gets brittle and can crack at the joints, which is exactly the failure you're worried about. The thing that actually settles it is the installation manual for your specific furnace. Lennox publishes venting instructions and tables for every model, listing exactly which pipe materials and cements are approved for exhaust and which are for intake air only. Building code essentially defers to whatever the manual says, so if the pipe isn't on that list, the plumber's position wins regardless of how the installer felt about it. Dig up the model number on the furnace rating plate and look up the install manual online. It's usually a short section and easy to read.

Also worth checking: look closely at the pipe itself. Some vent-certified plastic carries markings on the side saying it's rated for flue gas use. If yours is unmarked DWV-grade ABS, that supports the plumber's concern.

One more wrinkle. If ABS is on the intake side, that's harmless, since it's just pulling room air. The risk is only on the exhaust run. Gently flex a section near a joint. If it feels stiff, whitened, or shows hairline cracking, replace it with whatever the manual lists, usually done in an afternoon. If it passes the manual's list and still flexes fine, you likely have time to plan the swap rather than panic.

I'd skip the 'gently flex it' test the last reply suggested. My plumber buddy told me years ago that bending an old plastic vent run is how you turn hairline damage into an actual crack, right over the ceiling. The acid attack shows up worst inside the pipe wall anyway, so how it feels from the outside tells you almost nothing. If you alredy suspect the material isn't rated for exhaust, just price out the swap and do it on your schedule instead of poking at it. And honestly, a combustion check by a tech would tell you more about wheather anything is leaking than a hand test ever would.

Agreeing with the second reply on skipping the bend test, that was my exact fear when I found out my old furnace vent was questionable plastic. Mine turned out to be cellular core PVC, not even solid wall, and the HVAC tech I finally called just shook his head at it. What settled it for me was emailing the manufacturer with the model number and a photo of the pipe. They sent back a short reply naming the approved materials, and I printed it out so there was no arguing with anyone later. Took two days to hear back and it cost me nothing.