Not something I've run across before and I didn't dig too deeply.  It looks like IRC 408A uses a modified AGI that specifically excludes the income from Roth IRA conversions with no mention of excluding non-IRA qualified plans.  Live and learn.  Caveat: I did NOT follow into IRC 219(g)(3) to see what the other modifications were.

There was probably a better way for the client to do this.  So far all of the in-plan conversions I've seen (that, for confusing reasons, the IRS does refer to as both conversions and rollovers depending on where you look) have been conversions of after-tax contributions to Roth.  Big Tech (Google, Microsoft, etc.) have "Mega Backdoor" plans that permit after-tax contributions above the 401(k) threshold and they have an "automatic conversion" option that immediately converts these to Roth.  So the box 1 amount equals box 5 with 2a as $0 (prior to automatic conversions I would see small variances for <$5 interest each year).

But in your case I assume your box 5 is $0?  It sounds like this was a pre-tax 401(k) amount that was converted.  (Or rolled over.  Rollovered?)

The client can probably have the Roth IRA custodian do a recharacterization (to Traditional IRA) of the excess contribution.  If there aren't any other IRAs, this could be what I'd call a "delayed" back-door IRA, probably with a little bit of earnings.  So non-deductible contribution on the 2022 tax return and Roth conversion on the 2023 tax return.  But back-doors don't work if there are any other non-Roth flavored IRAs.  If recharacterization doesn't make sense, you're left with a corrective distribution (or paying the excise tax and kicking the can down the road).

Rick