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The rough opening is commonly a little larger than the finished opening. As long as it's properly shimmed, fastened, and insulated it is not a problem.
What exactly is your concern?
The gap looks very large up close and much smaller with the whole picture. I would ask him to install a full height strip to close up that gap.
The door can come down and be reinstalled after the shim strip is placed.
I would let a gap go to be shimmed up to 3/4". Over that, I would ask for a finish board of 3/4" thickness be installed full height.