No, there would not be a penalty for the estimated payments, as long as you take some sort of action. You simply need to handle the effect of the conversion, if there is any.

"If they don't withhold from the conversion and instead make a 1 time payment from their own funds to IRS now"

It's the same money and amount, either way. They can let there be withholding, then make up the difference to the brokerage for the conversion. Or, they can do a full direct conversion and avoid withholding, then make a higher June 15 estimated payment in consideration of that conversion's impact on their likely tax owed amount as you estimate it.

And you can look at safe harbor, too. Matching to safe harbor (based on the prior year) works well when someone's taxable income swings widely. Example:

They owe $1,000 in 2022, so they plan to pay $250 x 4 for estimates in 2023. In 2023 filing, you realize they are likely going to owe at least $10,000; but, they can meet safe harbor and deal with the balance when they file incurring no penalty. Of course, they still have to pay the other $8-9k when they file 2023, but there is no penalty if they met safe harbor in their estimate payments. That's why it is called Safe Harbor.

So, it works any of the three ways: increase the next payment for the one event, make up the conversion net to gross amount by allowing it to be withheld, or go with safe harbor for the year.