As a foriegn bystander your options are a bit limited, sadly. What you can do is disrupt the situation by asking the victim if someone is bothering them, walking over and showing your involvement, or shaming the harasser. Again, a good key word is "Yapma!" which means "Don't do that". If you feel physically threatened by the harasser but still want to help, appeal to others or scream to call attention to the situation.
