Is it cannibalism? That's like giving a bird some chicken. Or is it? These are the questions that need answering. And could watering your plant with caffeine beverages like soda or coffee or energy drinks possibly be good for it?
I think plants can only absorb certain nutrients in specific ratios or they will get sick. If that coffee has nutrients that the coffee plant can use it will only use those nutrients. If it gets too much or not enough of certain nutrients it will get nutrient deficiencies
Willfully or forcefully? Like ritualistic cannibalism is worse than opportunistic. Sliding scale there. There's no word for what you describe.. I hope.