I assume she's doing this because she hates the idea of wasted food. She can't bring herself to say, "It's okay to throw this out."
So do it for her--in that instance, *you* can "be the grownup" and throw the food away. She doesn't have to do it all the time. It's a vulnerability for her, almost a compulsion, not to waste food. So, just as you'd like her to occasionally and without much comment help you with your "issues," you can help her with hers!
Don't dump salt on it, hand it back to her, etc. There's no need to create drama, and you know that both of those will do so.
The "get involved" way to deal is to suggestion Grandma bring a takeout box from home/order or take less/whatever.
The "don't get that involved," simple way to deal is to just say, "Oh, nobody wants to eat this?" and then take it away from the table. You can even say, "I'll get this out of our way, then."