According to this Reddit post and a similar YouTube video, if you send a particular string of text to an iOS user, his or her device will restart.
The specific string of text for crashing the device is:
لُلُصّبُلُلصّبُررً ॣ ॣh ॣ ॣ 冗
You can hold your friends hostage for awhile, then give them the key to get the messaging app to play nice again:
- Launch the photos app, send a random photo to the person who sent you the text, then go into iMessage and delete the conversation.
- Use Siri to send a message to the person who just sent you the string. After Siri confirms and sends the message, open the Messages app and the problem should be resolved.
To prevent the problem again, your friend will need to turn off alert/banner message previews, or completely turn off message notification banners (Settings > Notifications > Message).