The All Time Low hitmaker had just wrapped shooting the promo for the band's new single I Found You when his pal revealed the shocking news.
Sykes tells British newspaper the Daily Record, "It was quite awkward, to be honest. I got back to my hotel after filming and my mate rang me and said, 'How was the video? Did you get with any hot girls?' and I said, 'Yeah, she was really hot. How's things with you anyway?'
"He replied, 'I just got a new girlfriend called Chloe,' I said, 'That's funny, the girl in the video's called Chloe'. And my friend said, 'Yeah, you've been kissing my new girlfriend'. Basically, all night I'd been getting off with my best mate's girlfriend."