Just a thought also, when Alfred first spoke to Harry, it caused him pain. Bonea was causing Harry's headaches. Maybe that was her trying to talk to Harry but lacking the proper frequency.
Mab knew this but says nothing because she wanted a lever to manipulate Harry. Alfred just doesn't think about these things unless Harry asks. Harry asked how to get Bonea out, not how to communicate. It took a meeting with Id-Harry to introduce her as something other than "the Parasite". Until then Harry didn't know Bonea was potentially a friendly.