Have your client take the missed RMD immediately.

Make sure the 2024 one is set up to happen before the end of the year.

Your client WILL get one From 1099-R for 2024, that has both amounts.

File F 5329 with the return (2023? or 2024?...) and request the penalty be waived.  

Prepare the client for a possible IRS missive; then you prepare a response:

"client corrected immediately, brokerage dropped the ball, system in place to make sure it doesn't happen again.

IF there's a penalty (the IRS almost always waived it in the past; there's talk they may now be less tolerant..no one knows...yet) AND there's proof the brokerage is the one at fault, then perhaps they could be held liable.  Or maybe be filling to write a letter to the IRS taking blame.  I have had that happen.

No, a preparer can't be penalized if a client ignores our advice.  I believe the IRS has the ability to track the omission down tho, and I'd guess they wouldn't be a forgiving if they find it themselves.

If a client of mine ignored my advise on THIS issue.... they'd be a former client.