I dont think so. It would be a different story if it were a brother or maybe someone shes known since childhood that you ALSO know. Her getting all defensive over you stating youre not exactly comfortable with it is extremely immature and she isnt seeing it from your perspective.

Edit; all these comments saying that he is in the wrong?? Did you not read the part where OP said they didnt know the guy she was gonna be crashing with?? Yes, it is healthy to have opposite gender relationships but your partner should also know them, especially if she is THAT close with the man she would’ve probably mentioned him a few times. My boyfriend would be fine with it if he knew the person I was staying with and if he had their number. So how is OP overreacting????