I have the SAME problem! Mom comes in and SIT DOWN ON MY COUCH! She takes off her shoes and everything. What I've started doing is (my living room - which is where the kids are during pick up and drop off hours - is right beside the front entrence) leaving the gate closed, having all his things ready to go - including his milk, and then picking him up over the gate and giving him to her. Then I walk away. Saying I am cooking dinner, or tending to something else. I always come back but she knows it's time to go. A few times she's even said "I guess we should go now". What does she think this is? A rest stop? Anyways, it seems to be doing the trick. I just don't pay attention to her conversation attempts. Luckily, she's sort of an awkward person so this whole situation makes her uncomfortable and she leaves.

Good luck!