All of the above advice is good. If there is time, she should get some other form of ID: a US Passport or US Passport Card (if applicable), a non-US Passport (if applicable), or a state-issue ID card. Personally, I use a US Passport Card with very little problem.

I have witnessed three times where people have come to the document checker and said they have lost or forgotten their ID. In each case they were asked to show several other identifying items, credit cards, student ID, anything with a name and photo on it. They were allowed to pass, but I saw that at least two of them got a more stringent pat-down. I assume the third did too, but I did not see it.

According to TSA's website, she should be able to fly domestically, with additional verification of identity. Allow extra time for this.