Ex Mil Is Threatening To Take Me To Small Claims (or)

Pretty much exactly as the title says. For background, my Ex MIL co-signed on an apartment for my ex and I. It was extenuating circumstances that my ex caused that made us have to move in the first place, so she paid for the deposit on this apartment as well. I moved out over a month ago and I refused to do anything with the stuff my ex left when he moved out. Petty, I know, but this person had put me through so much emotional and mental torment that I was not about to clean up another one of his messes. The apartment charged (ish, they took it out of the security deposit) my ex MIL about $800 for the disposal of stuff my ex left. Now, she is sending me an email asking for $400 or she is taking me to small claims. Prior to my ex moving out, we had no discussion about me removing anything that he left. There was no agreement in place that said that I would dispose of it for him. Additionally, I never received any notifications from the apartment that there was anything that they were withholding from the deposit. I never received notification that they did a walkthrough, itemized bill, nothing. I know that money is her money, but I did not agree to take care of this stuff. Does she have a leg to stand on? Would she be able to take me to small claims? The last thing I want is to have more contact with this toxic family, but I also do not want to shell out $400 if I don’t have to.