I saw someone mention about the 'meemaw cookie recipe' elsewhere & it got me thinkin too.
I too think it was Mary that was the 'messenger' between the two. Mary probably told Meemaw about Amy after she met Amy. I'm see both Sheldon & Mary know how important it is for Meemaw to like Amy, even, up until the last episode, if it was just through word of mouth. I'm sure Meemaw adorabed the sound of Amy which is probably why she allowed Amy access to her recipe (whether it was Mary who gave it to her or Meemaw essentially Meemaw would have had to give the go ahead initially) probably because she was so dead set on Amy being 'the one' for her moonpie she wanted to past on the recipe to the next Cooper women. That being her expectation that they'd eventually marry & be together forever built Meemaws picture of Amy up so high that when Meemaw heard about the break up, she marched straight to LA & shuned Amy the way she did.
It's all about the stakes in acting I know there are lots of people out there who hated Meemaws reaction to Amy but looking at it from Meemaws POV you can understand why she was so pissed.
Plus. It made way for sassy Sheldon *swoon*