I really don't think that etiquuette wise its a good idea to pretend that you didn't know who put the plant there and then throw it out, In fact, that seem pretty vindictive,
Also, a neighbor of 12 years who you know and are on good terms with is not a trespasser.
Having a good neighbor is a great thing even if she pkants something in your flower bed.
It's just a plant for crying out loud! It's not permanent. She didn't graffiti up the garage!
This has to be handled politely and with grace.
"Hey, Alice! Did you plant this in my garden?! I thought it sprung up awfully fast! I appreciate the gesture but its really not my style. Maybe we could plant it somewhere else or you could take it back, My garden is my own little domain and I only have enough attention for the things I have planted there. Again, I appreciate the thought, but I am kind of terrotorial about my little domain. What should we do with this new guy here?"
There really is no reason to be unfriendly. She overstepped. But, its a bad idea to turn her into an enemy because of it.