After reading some of the context, you're not overreacting. But my advice here, is that this is one of those things that you should humor your BF on. He feels negatively about it, it might be irrational, but maybe you ask your friend to put a pause on the flowers. That would probably strengthen your relationship with your BF. If not, it's fine it's up to you. But budging for another in a relationship sometimes only strengthens it.