VIDEO : Only One Direction for Niall Horan ... the Friend Zone

Our Ariana sources say the One Direction singer was invited to her concert in London Monday night. After the performance, she and Niall hung out backstage, but it was anything but intimate. After the backstage pow-wow, the group headed off to Cirque Le Soir nightclub -- and stayed out till 3:30 AM. Must be the wild video of their after-party that won 'em over??