I didn't find it unbelievable at all. Sometime in the future, gum is no longer a thing. It could happen.
A woman hands Dr. Beverly what she assumes is a mint or a candy or something. She starts eating it, realizes she's in trouble, but because she's a nice, polite lady, and the nice, polite lady who gave it to her is still right in front of her, she does her best to eat it with good manners and a smile, but she's obviously struggling to get it down. McFadden sells the shit out of it. As a nice, polite lady (sometimes) myself, it totally resonates. The shit we put ourselves through not to offend others.