First and foremost, Angels do not "turn to stone". They cease to exist when looked at. Second, it's not "seeing" that causes them to quantum lock, it's perceiving them that does. There is an episode in which a blinded character escapes the Angels by a form of echolocation to make them think they can be seen. Slenderman would have the same effect. Once they realized that he could tell where they were, they could quantum lock. Third, Angels in quantum locked state are indestructible (or at least they seem to be given that no one ever tries smashing them, and bullets don't seem to have any effect on them while stone).
In conclusion, Slendy would show up, quantum lock the Angels, get annoyed by them, then leave. They can't fight each other.
In conclusion, Slendy would show up, quantum lock the Angels, get annoyed by them, then leave. They can't fight each other.