Oh, god, Phoenix, there's no hope for you is there?!
Going by what your mutual friend told you (other thread):
• EITHER they did have a bit of a thing last year, OR your boyfriend got obsessed with her and she played along for a while, because he's her senior and a lot of young women don't know what to do with a creepy boss.
• I reckon he cut her out of the work project through spite. Maybe he'd tried it on with her and she knocked him back. I reckon she may have complained about his harassment then, too.
• The row your friend witnessed seems to be very much about HER telling HIM to back off. This makes it more likely he was harassing her, and she'd finally realised he wasn't going to stop.
• She is NOT chasing him, it's the other way around. He's spun this story to cover himself with you, and out of embarrassment.
• Work will have told him to shape up and behave professionally around her. He will have agreed. She probably thought it was all sorted.
• Then you turn up acting like the lunatic fishwife from hell. No wonder she was confused ("what have I done?") and upset.
• He shouted at her to fuck off because you wanted him to! He was just sticking to his story, that she chases him.
• He's lying to you, Phoenix. HE risked his job by letching all over her, getting emotional about her at work, refusing to back off. HIM. Do you get that??