Question about debt responsiblity after a roommate destroyed our apartment.

Without giving away too much personal information, I rented an apartment with my sister and a good friend. The friend got into drugs and refused to pay his rent, so my sister and I moved out early, turning in our keys and taking photos of the state of the apartment. That friend proceeded to destroy the entire apartment after we left, breaking doors and ripping up the carpet in some rooms and pouring hair dye on the carpet in others.

The rental company acknowledged that the damage happened after my sister and I left and therefore the friend was responsible, but included all of our names and information when the bill went to collections. Since then, the collection agency has contacted me and my sister multiple times. I explained the situation, what was discussed with the rental company, and provided them with the friend's updated information.

They outright refused to pursue him in any way, shape or form, and insisted that I am solely responsible for the cost of the damages. I've spoken to them over the phone multiple times and our interactions have ranged from unfriendly to downright aggressive and threatening(my credit specifically, though legal action was threatened as well), and now my credit is plummeting. What am I supposed to do here?