Going off of the last two panels, they did a lot more than fawn over him. How else would Dipper wind up with a hickey?
Mabel took Dipper along and actually made a move on the guy, and what guy would turn down hanging out with a pair of cute twins? Somewhere along Mabel's gameplan to divide, flirt, and conquer, she excused herself to the little girl's room and came back to the scene of Dipper pressed against a wall, head tilted back, and tittering while the boy got familiar with the expanse of "her" neck.
Mabel probably slapped the guy and dragged her too-friendly sister away, grumbling to herself while Dipper mouthed to the guy "call me."