You have a challenge on your hands. It is, even if the idea seems daunting, much easier to access the plumbing fittings from the other side of the wall. If there is no access panel in the wall in the adjoining room, you need to make one. This is helpful if any of the fittings need to be replaced in the future or if the drain gets clogged it provides a much easier place from which to snake or jet the drain.
In your case it will be necessary to access this to remove the barrel and rod unless you can grab them with a flexible grabber of some sort. But even if you can, you more than likely won't be able to remove it to replace it with a new assembly. If you can and it is still intact you could re-attach it to a new plunger lever assy. If your plumber has never had this happen, he hasn't been a plumber very long! How old is the house? It may be well worth it to replace the fittings.