Barty noted the couple standing up to leave but gave an inconspicuous gesture to Chas to stay put. No sense in spooking the pair into knowing they were being followed, after all. The element of surprise was going to be more fun than chasing a Muggle in this case. He took a slow sip from his coffee as the couple paid and left and let them get a head start before he tossed down some Muggle money and and then stood to follow not far behind.
He didn't wait for Chas to follow his lead because he figured she'd take the cue and be right by his side.
The couple stopped to window shop just ahead and Barty smirked to himself as he came up behind the woman, raised his arm and closed his hand over her mouth.