If Qui Lin weren't there to save Cang Cang, the tragedy would happen :
Case 1) the guy must have a knife with him for sure, he could stab Cang and her roommate then do the-disgusting-thing all the pervert will do to the death body of Cang. If it were a normal girl, this case would happen. But it is an excellent Cang Cang, she could find a way to save her life.
Case 2) Cang were calm enough to think a way to espace from that guy while he were take off her clothes or such. She could find a best timing and run away to call help and later the guy would be caught by police. With Cang Cang's excellent brain, this case for sure happen.