Billy did have a girlfriend at the time but Isabelle had never made it feel like she was trying to steal him away. Maybe that's why he didn't notice really, because she was just flirting to have fun and not to get anything back from it.

"Dude, shut up, Alec," Billy muttered under his breath, although definitely loud enough that Isabelle could hear. He appreciated his friend looking out for him but Billy liked how Isabelle made him feel. "I mean... I have issues getting physically close to people unless I know them really well but the way you make me feel "uncomfortable" is a good way. Like I feel so much good stuff I don't know what to do with it. It's nice."