I think your partner is partly to blame for her behaviour. If he knows you feel uncomfortable with this, then he has a duty to discourage her from turning up at your home when you're not there.
If she turns up uninvited, he could put his jacket on and tell her he's on his way out to pick you up and ask her to phone first, as it's not always convenient. If she's a genuine friend and not just meddling in your relationship, for her own sadistic pleasure, she will understand.