The serial celebrity dater, who's bedded the likes of Jessica Simpson and Jennifer Aniston, chased Katy until she finally gave in … only to dump her a few weeks' later.
"John did a real number on Katy," a source tells Radar Online. "She held off from his advances for the longest time, but he laid on the charm thick and promised her he was a changed man. She eventually gave in, fell head over heels for him and then, yep, once he had made his conquest, he dumped her."
The former couple attended a mutual friend's party this week but completely blanked each other for the entire evening.
"It was so weird!" an eyewitness told Radar Online. "Katy stayed in one room the whole night and John stayed in the other and when they passed each other they both just kept their heads down.
"It was like they had this incredible animosity for each other and everyone else at the party could feel it
"You could cut the atmosphere with a knife. It was all incredibly high school!"
Country singer Taylor Swift famously penned a song about how awful John was after he dumped her. Maybe Katy will do the same now.
A hint, John: stick to dating actresses!