According to Bulbapedia, it does, however it does NOT work on showdown, so can somebody test it in-game please?
To be more specific, lets say Florges is in a double battle, allied with Krookodile.
In the specific situation that didnt work in showdown when I thought it should have, Florges uses moonblast and is holding an assault vest (its a weird strategy OK?), and then Krookodile uses Fling, and flings its king's rock, and then nothing happens. The opponents used regular attacking moves with no relevent secondary effects, though I dont remember what they were specifically, so I dont think they had anything to do with it. I checked to make sure the ability actually works by giving them both berries, and when Krookodile ate his, florges gave him her berry. Is it perhaps impossible for symbiosis to pass an assault vest? Or does it not actually work with fling? Or maybe showdown has the mechanics wrong?