It's hard to know for sure without a picture but it doesn't sound good. His application technique may be to blame. You're supposed to cut the sucker back low and use an old mustard bottle or something similar to put a few drops of the chemical on each cut, not spray it. I don't know how you could avoid getting it on the tree when spraying unless he'd wrapped the trunk beforehand. Cut out the dead wood and see what comes back in spring, unless it's too ugly to deal with.

I have a love/hate relationship with Canadian chokecherry. I love the changing leaf color and the scent of the blossoms, but it's not a tough tree, seems very prone to insect and disease damage, like many fruit trees.

Maybe this is an opportunity to amend the monoculture and plant something different in it's place.

Barb

Barb, thanks for your reply. I think I've convinced him to wait until spring to decide what to do with it. I told him to just apply the poison per the instructions to the cut ends of the suckers, but he isn't much for reading or following the instructions:(