Ahhh, the weaker sex strikes again! This time it they were not weaker and it will take you a week to fix!
Seriously, bummer to hear about it. Doesn't the vehicle have locking gas flap? I am not even going to ask how she could have gotten it in there without your noticing. It CAN happen!
The gas tank is not in an easy thing to replace. As difficult as it is to get to, it may more difficult to clean an old tank, depending on what "chemical" was used by your X.
You may find a tank from a junk yard to be another option.
None of them seem like good choices. Better to fix the relationship or move on so it does not happen again.
Crazy CAN LAST a lifetime, and….Anyone CAN MISS their medication.
Sorry for your loses.