Is a flexible coupling okay to use on an ABS furnace exhaust pipe?
The short answer is that you'll have to consult your manufacturer's installation specifications. 

Some furnace manufacturers specify only Schedule 40 PVC. However, some water heaters ship with couplings intended for use on the exhaust duct, so it's probably not accurate to say that flex couplings can never be used. 

If you find that they're allowed, I'd probably use a shielded (metal-wrapped) coupling, especially since this is a horizontal application. You don't want the rubber carrying any load.