The mechanic gave you sound, conservative advice as he should, and while its possible the rim may fail, I have fixed dinged rims by bending them back in line, and then filing the braking surface so the ding doesn't catch on the brakes, with no catastrophic results.
If you think the rim is so bent it won't hold the tire, pump the tire up to 10-20 PSI above max momentarily and if it holds, you should be ok.
Il faut de l'audace, encore de l'audace, toujours de l'audace
1980 3Rensho-- 1975 Raleigh Sprite 3spd
1990s Raleigh M20 MTB--2007 Windsor Hour (track)
1988 Ducati 750 F1