Unfortunately, as you have experienced, a co-signer on any kind of debt is liable in the event the other signing party defaults. As such, you are liable for the other party's lack of fiscal sanity and dereliction of duty. There's really no option for you at this point except to pay the amount that you contractually promised to be liable for.
As far as the apartment manager is concerned, unless the lease agreement obligates him to contact the co-signer if the resident fails to pay rent, he is without fault. That's not to say that an attorney might be able to show that he did have some duty to you, but that might be expensive to prove and would probably only apply to a portion of the unpaid rent.
I'm sorry to opine that you might be on the hook for the entire unpaid amount. Your only real recourse is to file suit in Small Claims Court against the party you co-signed with.
Did you sign as a "guarantor" or as the tenant? If the lease states that you live in the apartment, the landlord and its attorneys my have tried to contact you at the apartment and your friend may have neglected to forward any communications to you. You should verify the amount of rent and damages owing to the landlord, find out how much your roommate paid and how much your roommate is willing to pay, and then negotiate with the landlord and/or landlord's collection agency. If your former roommate has no money, the landlord can come after you, put the account on your credit and possibly list you as having an eviction on your record. If you have an eviction on your record, you may have difficulty the next time you want to rent an apartment. It might be worth your while to meet with a landlord - tenant attorney because, sometimes, an eviction and a negative credit report can be removed from your record. You may want to try to negotiate this if you are going to be settling this.