Plumbing question - a little long but need advice

We just moved into our coop last week, and last night we encountered a plumbing problem.

We I did my walk through with the inspector, he found a couple of things that needed to be fixed (a leak under the bathroom and kitchen sink). The seller apparently fixed them. Did final walk through before closing and ran the water for awauile and didn't see any leaking under the sinks.

Well, last night my husband decided to take a bath, no sooner did he turn off the water my neighbor downstairs came up and said there was a leak in her kitchen. We contacted the super, he cut a hole in her ceiling to see what the issue was. Well, there isn't a continuous leak, but he came to find out that the seller did a crappy job installing the bathtub when he updated it. They forgot to install an item that goes into pipes that absorbs any excess water from baths into the pipe, which is why there was a leak last night after the bath.

Now, my question is are we repsponsible for paying for these repairs. This wasn't anything that we could have picked up on the during the walkthrough with the inspector b/c anything in the walls is the coop responsibility, we weren't taking baths, and our downstairs neighbor wasn't home. But, they are telling us that b/c this guy didn't use a licensed plumber and went behind the coop's back with this that we are responsible. My attorney said that we can't sue the seller b/c this happened post closing. We homeowners insurance, but I don't see how this is our fault.

Thanks for any advice.

Re: Plumbing question - a little long but need advice