Plumber at fault, what next?

[Matt Browne]

We have been working on renovating a multifamily home (Midwest) and had a large chain plumber redo our water lines. Four months later, we find out that we have water leaking from our third floor onto our second floor causing damage in the walls and bathroom. We are also starting to see mold. 

It was confirmed to be one of the lines from their big project. We found this out with another plumber and got photos and videos of the damage.

We called the first plumber back since the line is still under warranty. They fix the line correctly this time, but don't want to repair the wall damage. Told me to call corporate who is not returning my calls. What should we do to get our bathroom back in place? Especially since they are at fault. We also have a tenant in the 2nd floor unit that is being affected.