Ask her not to go with that guy. You're right, you can't stop her from going to the concert and shouldn't. Ask her to not go with him. There's something weird about it and you're uncomfortable with it. If she decides to go with him....still nothing you can do, but at least she will then know you have reservations about this person AND if there is something going on.....then she knows you have your suspicions.
Don't tell her you don't want to move out. Tell her you are NOT going to. End of topic.
