She should have broken it off with him when they had the argument in the stairwell.

He’s clearly more into her than she is into him. When he says “I saw we’re a couple, dammit!” And she agrees? It’s at that point she made a decision.

She didn’t want anything serious? Cool. This is the jump to “serious”. She is leading him on after this. If her heart isn’t in it, the decent thing is to break it off.