Were The Flywheel Ring Gear Teeth Actually Inspected Or Is The Flywheel Assumed To Be Shot ?
The way to destroy this gear is to continue to let it intermittently grind when a starter is failing. They have to be repaired / replaced ASAP. A screwed up flywheel can also cause starter problems.
The car possibly needs another starter and may or may not need a flywheel, but it likely does by now. Sometimes if just slightly burred they can be "cleaned up," instead. Now the starter should come off, be tested / inspected, and the flywheel gear inspected.
I'd get some other guesstimates for diagnosing and / or replacing the flywheel. Try a couple of local reputable independent shops or local community college / high school auto class.