I'm worried and I don't want my friend to bring someone that no one knows with him while we go out
That can be a little scary. Why don't you want to see this stranger?
Because I don't want to be around an unfamiliar person.
Think about it this way. How do you ever meet new people if you're unwilling to hang out with anyone you don't know? Maybe your friend just wants to introduce you to someone fun and new.
I don't have any interest and it's pointless.
That's okay then, you don't need to be trying to meet new people every time you go out. But this is your friend, don't you trust his judgement that this stranger is worthy enough to tag along? How about just giving him a chance this time around, you might make a new friend.