To summarize, it seems that the account is now in a state where it cannot be accessed because it is disabled, and appealing the ban requires logging into the account first, which is impossible. I think it is possible that Facebook did not consider this flow, and there is no provided way to resolve it.

Attempting to log into the Facebook account explains that it's disabled because the linked Instagram violated the Instagram community guidelines, and a link to the Instagram help center is provided, but it can't be used without logging in.

I also found this Facebook form: https://www.facebook.com/help/contact/260749603972907 However, attempting to use it results in the message: "Log into your Facebook account for more information about how to request a review."

I'm sorry for the noise if there is something I'm missing here. I would really appreciate any advice or assistance. She is a small business owner who relies on Facebook because that is how her clients wish to interact with small businesses, so obviously she is really in a frustrating position currently.