I'm going to warn this guy about his date
Why would you warn him about his date?
Because she bit an ex on the face over an argument. He probably doesn't know.
It's good to warn people if they are in danger. That way you can make sure they're careful and possibly cancel the engagement if possible.
He can just find another date.
Right. If he feels she's dangerous, he can drop her and go to someone else. Good for warning her.