Luciano Spalletti's side were 2-0 ahead with ten minutes to go. With four matches to play this season, now instead Napoli are five points behind leaders Inter Milan and four adrift of second placed AC Milan, who both have a game in hand.
Dries Mertens opened the scoring just before the break with Lorenzo Insigne adding a second eight minutes after the interval. But Napoli's defence collapsed in the absence of the suspended Kalidou Koulibaly.
A Kevin Malcuit blunder allowed Henderson to pull a goal back after 80 minutes with Andrea Pinamonti making the most of a sensational error from visiting goalkeeper Alex Meret to grab the equaliser after 83 minutes. Pinamonti added a second five minutes later for the 14th-placed hosts to end their 16-match winless run going back to December 12 when they beat Napoli 1-0 in Naples.