It isn’t. However, it is slightly ridiculous when it would perfectly solve his dire problem. Additionally, it isn’t like he is giving the password to a total stranger. It is his girlfriend living in his house. If she really wanted to screw him over, she already has that capability.It’s not ridiculous to want privacy and not share passwords.
HomeKit solution will most likely end up in with a home reset or you traveling, smoke.
One wonders if there is a reason he doesn’t want her to have it… 🤔