In Plan conversions gone crazy

I have a client with a solo 401(k) plan. Upon the recommendation of his CPA, he converted about $400,000 of pre-tax plan monies to Roth. The problem is that instead of moving the funds to a Roth account within the 401(k) Plan the funds were moved to a traditional IRA, then converted to a Roth IRA, where the funds currently are being held. The taxation for 2025 is correct and the funds are still invested. We explained to the client that this was an ineligible distribution and the funds need to be returned to the Plan, adjusted for earnings. When they reached out to the Custodian to request that they needed to "undo" it all, they stated that they can't move the Roth IRA funds to a Roth 401(k) account because Roth IRAs can only roll into Roth IRAs. Any suggestions or insight on how to get the Custodian to correct the error? Outside of "undoing" it, what corrective options do they have?