It was a Citi card, no Amex involved thankfully. I have statements from Citi as well as my Mint transactions, so I have record of how much I've spent and paid.
I talked to my ex. She took it surprisingly well. She recently got a job offer so that will aid her in paying the bill. It was difficult for me to hand this off to her, but I wish her luck.
Looking at the bright side of things at least you had the respect to let her know.
Many would have just said who cares what happens and stopped paying.
Good Luck in your future...