Yeah agreed.. A little subtle acknowledgement, to politely say hi.. But not to draw attention..
Of course if shes a friend.. Saying hi is fine on the basis shes working.. I used to be close with many girls when I was single, and often would see them with other guys, but they were actual friends so saying hi then seems entirely fair.. You might get some idiot get bent out of shape about it but your saying hi not taking her off..