I panicked. In this situation I would normally pull the database out of the AG and then re-add it. I didn’t have that option because it is a HUGE database and didn’t have that much time and space to move it around. I knew a large transaction had kicked off (thank you alert email that I created to warn me about such things) but thought that surely the rollback would have cleared quickly. That lead me to looking for rolling back transactions.
Fortunately, the issue was on a secondary not under heavy use, so she was able to recover in time.