Nothing wrong with carrying a BUG. However, if something goes down and you need to arm someone else, I think the last legal worry on your mind will be arming someone unlicensed. You are going to be dealing with the aftermath of shooting another person. That's going to steal the limelight away from any lesser concerns. You also would probably rather increase your odds of survival than avoid a comparatively minor crime. And you could always need the BUG yourself.
I would recommend a revolver or other DAO pistol for this purpose. It may not be your SO you are arming, it could be a stranger. You want something that pulling the trigger is all you need to do, in case the person is not familiar with guns beyond what they see on TV.