I don't believe the mechanic.
This breakage is not uncommon due to torque values being EXTREMELY DIFFERENT on either side of tranny for those fixtures, and mechanic makes big mistake.
If you buy a used tranny you are going to overhaul it anyway. So either a used tranny or used case will cost you the same in the end.
Check with mechanic about torque specs for those fixtures and ask what he use. Don't tell him, but look in manual, I believe there is over a hundred pound difference, Yes that much or more!
Shop is bad and a liar.