I would simply let her know that at this age if mom pops in and leaves without him it could create a bigger anxiety problem. He's left in a new place, mom comes to "visit" and leaves without him might be very tramatic to a little one that doesn't understand.

The lesson we need to teach him is ... mom always comes back and picks you up. Popping in to "check" on them is completely contradictory (sp) to that lesson.

The other problem is, a parent who thinks it's ok to pop in once, pops in often! Who wants to added stress of wondering if today mom will pop in or not! It upsets everyone, not just her own child.