Until the land lord's judgment is satisfied, both lessees are responsible. You were originally sued by your friend. Since the lease is still your joint responsibility you are both named.
You can file a response saying you were already assessed and paid the money. This is called res judica. Mention the case number and date it was heard.
OR, if in NJ an appearance is a response,
When you appear in court, point out to the Judge the case number and the outcome. If you have a copy of the decision even better. Also point out that you have paid (or are still making payments).
What you want, or need to do, is remove yourself from this case. That is the limit of what you can do. You have no cause to sue your ex friend.