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As fast as that starter would be spinning I would expect a fair amount of noise... but you never know how observant folks are or if they'd be able to identify the source nor do I know how much he's been driving it since this started. Just throwing the possibility out there. For me it'd be a fairly quick operation to just pull it and check for any damage, but I don't send work to a shop for such things.

I may be wrong but wouldn't an engaged starter generate voltage which could be tested for?

Anyway, as I said, this scenario isn't very common as far as I know (at least not in anything since the made 80's; See: older domestics, esp. Chevy starter shim fix) and isn't likely to be the problem. I figure it is worth a quick check since kobesupra's brother insists that all he did was crank the starter while the engine was running. If this did happen I wouldn't expect the starter to function for long before the bearings start screaming and die a terrible death.

If there are diagnostic codes present which the mechanic is dismissing you should have him write them down and post them. If he won't get the codes read at an auto parts store (every one around here will read codes for free). I suspect that by saying the codes are "normal" he may have meant there were none present but it's worth verifying.