"I am specifically inquiring about the Federal Income Tax that was withheld from her distribution and entered in box 4 of the 1099-R. Are we allowed to split it ratably over 3 years as well?"

it's no different than an Estimated payment or any other amount withheld, now. If she owes that much in taxes, even setting aside the covid withdrawal into future years, then it isn't returned. However, if she later decides to "put it back" per the provisions, she would need to make up the withheld amount or has a taxable distribution. I've seen this happening, where people don't understand that "putting it back" is supposed to be the same gross total, to avoid having a taxable event.