What would happen to my son if I reported the forgery?

My college aged son, who is on an athletic scholarship, wanted to rent an expensive apartment with a friend, and wanted me to cosign. They would move in August 9, 2017 after the current occupants move out. I had signed a cosigner background check application nonspecific to a particular apartment. I told him to find more roommates to bring the cost down, or to find something cheaper. He didn't, and then on 5/19 forged my signature as a guarantor, which holds the guarantor responsible for everything. On 5/23, I emailed the landlord stating that I rescind my signature as either cosigner or guarantor on this apartment lease. At first she accepted this but later she insisted that I had agreed to being a guarantor (never did-it was forged. I mentioned to her that the signature on the form that she sent me via attachment wasn't mine). The boys had had to sign a Cancellation Form if they ever wanted to cancel the agreement before moving in, agreeing to forfeit some of their ($2500 total) deposit. To pay for his share of the deposit, my son used his balance on a former (expired) lease with the same landlord ($1000 that we had paid in January 2016) as well as a $250 check he wrote that bounced. Currently, he and his friend have agreed to cancel the lease and have signed cancellation letters. They will lose $1000 of the deposit. The landlord said that they also have to find replacement tenants. Now the landlord is going after my son for the $250 check that bounced. She says that she will take action if he doesn't pay the $250. Do they even have a valid lease agreement? Can she come after me?