Did this happen because your client changed employment during 2021?

If your client failed to notify the plan administrator of the excess and to request withdrawal (including any allocable earnings) by April 18, there are some punitive tax consequences.  Elective (and designated ROTH) contributions will be subject to tax both in the year of contribution and the year of distribution because the contributions will not be treated as having a basis.  This would mean the income would be subject to tax twice and early distribution penalty of 10% will also apply unless one of the exceptions is met.  What's more?  Late withdrawal may not be possible until the next plan permitted triggering event.

If your client did not change employer and it's that one employer's plan administrator who didn't catch the excess by April 18 to take corrective actions, the failure could potentially cause the plan to be disqualified and subject the plan to certain reportable scrutiny.  This means the employer's plan administrator would have more skin in the game to get this fixed ASAP.  Unfortunately, your client will still be on the hook for the tax consequences explained above.