custodian of my husbands IRA failed to execute his order to do his RMD in 2024. they sent a letter admitting to it. how do we escape penalization for this?

Yes, you must file form 5329 but I would send a copy of the letter and keep the original. I have another post with step by step entry all the screen shots to ask for waiver. 

If you want to send the custodians letter, you will need to print and mail your return. I have another post on mailing a return.

Reference: About Form 5329, Additional Taxes on Qualified Plans (including IRAs) and Other Tax-Favored Accounts

Did you e-File already a tax return with a penalty?

The process is different depending on the answer.

Generally, IRS does not require the custodian's letter which would prevent e-File.