If the buyer paid with PayPal, they have 180 days of coverage for items that are not as described.My friend has this problem with her iPhone 6 Plus.
I inserted double sided tape between the logic board and the rear case to push the Touch ICs up against the logic board.
The phone seems to be working for now.
She sold it on eBay and just shipped it.
Hopefully it last long enough to pass the 30 days return mark.
You should tell your friend to disclose the bandaid repair and allow the buyer the option to return the phone. That would be the honorable thing to do.