Does my friend have to pay the rent owed or can the ongoing problems with the house makeup the difference?

My friends live in a house that I used to live in with my boyfriend. When I was living there, it was rundown already. We always asked the landlord to fix things but it never happened so we moved out. My friends were in need of a place fast so they signed a lease with our landlord and moved in. The house is in shambles. Mice get in constantly. The electric outlets spark. The front porch is falling off. The back porch door fell off the hinges. The bathroom floor all around the toilet is sinking in. Water leaks from the bathroom floor through the kitchen ceiling. It's a horrible house. Anyways, my friends are 2 months behind on rent (approx. $1600) and they just got served with papers that the landlord is suing them for $5000. I told my friends to go to court and fight it. We have doents proving we contacted him to fizx things. Do they have a case against the landlord for not paying rent? Can they sue him for being a slumlord? I think it's ridiculous what he's doing!