A glance or casual look is no biggie. I am capable of appreciating the good looks of a guy OR girl within the time frame of a casual glance.
However, if he stares, turns his head to continue watching her, or drool forms from his mouth, we are gonna have a major issue. Matter of fact, if any of those things happened (stares, head turning, drooling), I'd approach her myself & ask for her number for my newly single friend...just to prove a point.
It isn't about jealousy, it's about not being disrespected. It IS disrespectful to check out another person when your with someone.
That should be common sense.