99% of the time a coil insert, which calls for very little additional loss of crank arm material will work fine. I don't know of any failure of a crank arm at the pedal thread end because of added weakness after inserts were installed.
I suggest finding a different shop, or better yet a bike co-op (I'm fairly sure there's one in San Diego) and having it done.
The 1% of the time you cannot coil a crank, is when too much material was remover out as the pedal tore out under load. This is rare but does happen, making fitting a coil problematic.
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