Homeowner has plumbing issues on the flip I sold...PLEASE ADVISE

[Huso Akaratovic]

Greetings all, 

I am in a unique situation and I am not exactly sure how to proceed with it. Here is the story. I sold a flip house approximately a little over a month ago. The house was sold FSBO with no agent on the other side either. The home inspection passed and so did the appraisal without any issues. The buyer called me last week and was asking me where the main drain on the house is. I also wasn't aware where it was. He then proceeded to tell me that his water had clogged up throughout the house and the plumbers were on scene handling it. I felt bad for him, but there was nothing I could do to help him. While I had the house in my possession everything was checked out by my plumbers and determined to be In a good working order. I had no knowledge of any issues. On the positive note, I paid for his home warranty and advised him to use that to cover the expenses.

Today, the buyer texted me. He stated that He is having issues with the plumbing and the plumber came out for the second time. This time the plumber told him that the sewer pipe collapsed and it would cost $6200-$7800 to replace it. He stated that  he called the warranty company and they advised him that this item will not be covered on the warranty. He than stated that after having a discussion with his wife  they believe that I should be responsible for paying half of these expenses. I am not really sure what to do. I haven't responded to his message. Can I please get some input? Thank you in advance for all the responses.