This is a semi-educated guess, but I think it may be that she's not guarding you so much at protecting her spot in the pack. If she's sleeping beside you then she's you're equal. When your boyfriend comes along to wake you he's a threat to the pecking order. Tucker doesn't get involved because he's below you and as such, below you and Biddy.
I'm only guessing this because I've been told this is the reason not to along your dog to sleep in your bed. If you're the leader and they get to sleep beside you, then they are an equal in their minds.
Then again, I could be completely wrong
Tom