I'm not much of a prankster, nor have ever done this one, but nearly my friend's entire extended family goes nuts with this.
When your friend leaves you for a moment, while leaving their cell phone in front of you, go into the phone list, and look for names like "Mom", "Dad", "Boss", etc. Leave the name alone, but edit the number to your own.
Later that night, send text messages that read anything like:
"I can't believe you f*@#$& did THAT. What on earth were you thinking!? I AM SO ASHAMED OF YOU!"
- Mom