The Proper BB thread cutting tool is special to bicycle Manufacturing . it has a Guide pin so both threads are in alignment precisely.. Park Tool Co. » BTS-1 : Bottom Bracket Tapping & Facing Set : Frame & Fork Tools
(as a Charity with valid 501c3 status you may be able to offer a Tax write down to a Bike shops Business accounting , to get the Shop to do the work with the proper tool.)

A BB which does Not Use the threads was made and is sold by Velo Orange Grand Cru Threadless Bottom Bracket - Bottom Brackets - Components


on the cheap and not fixing the problem .. but making it function.. the shimano UN type BB with a plastic Left sleeve at least wont make things worse ..

an Epoxy filler, cured. then cut with the BB tap, may replace lost metal somewhat .. [JB weld has aluminum, Powdered, in its resin Portion]
at least disclose the damage and be willing sell the bike, with that information, for Less .