So my dad passed away a litle over a month ago out of country, and we're currently trying to file his life insurance policy claim. We're required to fill out a W-9 as well, but since my mom, the beneficiary, is undocumented, we didn't want to claim her as a U.S Person on the form. The insurance company reps said to fill out a W-8BEN instead.

After some research, apparently tax laws and different from immigration laws, and depending on either the green card test or the substantial presence test, even illegal immigrants would be classified as a resident alien. Tax law only have 2 categories for tax payers, resident or nonresident alien, so I'm not sure if resident alien would count as a U.S Person in terms of tax law. Anyone have any ideas on this?

The tax forms are probably going to be used on taxes on the interest accrued from the proceeds, so not sure which one to fill out.