Due to the fact that you can only snap people that are in your Snapchat list, this kind of behavior isn't accepted by Snapchat users. It this case, I strongly suggest that you contact the developer and provide an evidence of the place where you found the other persons username. If they made their profile public, it means that they wanted to be contacted by strangers. Also, check the following support website for Privacy Policy that you both agreed on when you installed the application.
