"What does "more documentation" mean? what exactly are they asking for?"

They want to see her W-2s and her tax return.

Regardless, more hurdles to jump will ALWAYS lower the percentage of eligible people enrolled.

"Lemme guess ... you think that's asking too much?"

No, not at all. As long as they weren't chosen for scrutiny because, say, they're a poor person who was gifted an iPhone.

Literally two days ago, I had to cut out actual W-2s and return them to a tax client's home state to prove state taxes had been withheld. It could be for any number of reasons why the state thought he had zero withheld instead of $8K, but none of them were ours.

Which reminds me: if a taxpayer files a paper tax return and the IRS loses it, that's legally on the taxpayer, who would then be responsible for substitute proof.

"what exactly are they asking for?"

Ideally, anything that doesn't pass the smell test.

Is she wearing designer clothes? Is she driving a new $80,000 car? Does she live in a neighborhood where the property taxes are more than most people's rent?

I've seen where people have listed their monthly bills, and that amount was considerably over the income they were claiming.

Frankly, investigating stuff like that is exactly what I hope happening.