I'm trying not to take sides here since there is only one side of the story (although corroborated by a witness) - but I'm assuming the garbage was found during boarding, when she tried to stow her water bottle into the seatback pocket? If the FA refused to take the garbage during boarding, couldn't she have thrown it away in the lav, or just left the garbage in the pocket?
And if it was wet garbage, why would you even want to stow your water bottle in the seat back pocket after discovering it?