Exactly what you need to do probably depends on how your condo agreement is written and the laws of your state, so you need to check those carefully. If your neighbor does sue, you will need a lawyer.

Your story is a bit confusing. Some points need clearing up.

1. You replaced the p-trap. Was this on the kitchen sink? And because of this one change to one sink, the building management is not responsible for any water problems in your unit, even water problems caused by the toilet or bathroom sink?

2. Has the leak occurred again or was this a one-time thing, perhaps caused by the work on your bathroom, which created a problem that was fixed the next day?

3. Could you have been the one flushing at the time the tenant states she heard the flush? or were you out of the unit or in bed at the time? In other words, could the flush, which may or may not be the source of her leak, have come from your unit?

3. Have you had an independent inspection? I have to admit that your unit would be the first place I'd suspect of causing the leak, because of the proximity and the work done on both kitchen and bathroom. But water is a funny thing--it can travel in ways you wouldn't suspect in buildings. So while the leak might be caused by a leak in a pipe you can't see, it might also be coming from a unit above yours or to one side of you.

Thanks for your reply!
1- Yes we replaced the p-trap on the kitchen sink. And beause of this one change to one sink, the building management claims that they're not responsible for any water problems from our unit. we were under the impression that since it's a pipe that is NOT in the wall, that we could change it.
2- The leak occurred only once- about 2 weeks ago, and we just heard of it yesterday when management told us about it.
The bathroom that shares the wall and pipes with the kitchen has not been used. the toilet and vanity have been removed and pipes have been closed off. we've had open walls for over a month and see all the pipes. no trace of water or a leak of any kind.
3- we havent had an independent inspection yet. from my readings on the web, the neighbor should first file a claim with her homeowner's insurance company...who them will investigate further and decide if the damage was due to our negligence.

I'd suggest that the best defense is a strong offense. Go downstairs and talk to your neighbor rather that letting her stew. Ascertain the extent of the damage, which sounds like a little wet drywall. Replace it and paint for a couple of hundred dollars and be done with it. If she wants to turn it into something more, tell her to call her insurance company, which in turn will call your insurance company and they'll work out the details.

You might want to pay a licensed plumber to check your work and look for any other potential problems before you button up the walls.

we tried to go downstairs to see the damage but she wouldn't let us in at 8:30pm claiming it was too late and she didn't feel comfortable letting us in because she was home alone.
my husband is a plumber. we did absolutely nothing wrong on our end nor did we cause any leaks. the blueprint of the building's plumbing would confirm that we aren't the only ones that could have caused this leak to the floor below.
i appreciate the input. thanks again! :)

Well, call her up and arrange a time that is convenient for her when she can have a friend there. IMO, she does get to call the shots on who she lets into her home and when. I can understand a woman home alone not wanting to let two relative strangers into her home late in the evening. But that doesn't mean that you absolutely can't get in to look at the damage--just that you have to accommodate her needs.

Also, ask the building management for a copy, in writing, of the inspection report that determined your apartment was the cause of the leak. Either there isn't one and they just guessed it was your place, or there is one and you have a problem you didn't know about. But you need that info in order to deal with the whole situation.

And I'd check the co-op agreement very carefully. There should be some instructions on how to deal with this sort of problem in there. And also, there should be language about making changes like plumbing changes and how that affects what the building management can or can't do. You may want to scale back your remodeling if it will mean that you will be responsible for any and all damages now and in the future.

The building didn't do an inspection determining that our apartment caused the damage. When i asked their answer was "it's not an assumption. your apartment is the only place it could have come from". then, they simply looked at our kitchen and saw that we changed the p-trap under the kitchen sink and just assumed we are to blame. a look at the blueprint would clearly show all the possibilities that the water could have come from.
We read the co-op agreement and by-laws and it doesnt state anything about making changes to plumbing nor does it explain what we are responsible for maintaining and what the co-op is responsible for maintaining. when i reached out to management with this question, it was never answered but simply danced around. at the walk through, we were told anything behind the wall is the co-op, anything that is seen is our responsbility. the p-trap is in my cabinet, we see it, it's not hidden in the wall by any means. since my husband is a plumber, we didn't think anything of it when he swapped it. there hasn't been one drop of water on our end to make us think it was installed incorrectly and we've been using the sink and dishwasher for a couple of months.
I guess we have to wait and see if the neighbor pursues anything. Once her insurance is involved, i am sure an inspection will need to get done to determine what caused the damage. but until then, i refuse to simply take the blame and pay for her damage.

thanks for all your input!