Posting defamatory or potentially defamatory remarks about an identified or identifiable person on any social networking site is not only a violation of the Terms of Service of these sites, which can result in the termination of the account and a barring from the site, but it can also result in a lawsuit being filed against the person who has posted the remarks.
The only defenses available to a poster of defamatory remarks on MySpace would be that the remarks made were true or the remarks made were pure opinion that did not imply any false fact. From what you have posted here, opinion would not fly as a defense, and truth would be hard to prove.
There would be no jail time - just a potential loss of a GREAT deal of money (many many thousands of dollars), should this MySpace poster be sued and lose the suit.
Rumors that have the ability to affect the reputation of an individual or individuals should NEVER be published unless the rumored statements made are true or have the good chance of being proven true.
Whether any legal action will be taken or not is a question none of us can answer. The poster may be asked to remove the material, however, coming in the form of a cease and desist letter from an attorney. And the poster should probably remove the material before such a letter arrives, to mitigate any potential damages if a suit arises from this.
Many people will threaten lawsuits, then decide against them when they see how expensive they are to pursue. Educators in many states, however, have brought suits against students who have defamed them online, and they have won these suits - leaving the students with angry parents when the parents' assets are tapped to satisfy a judgment, and leaving the students with a court record that can hurt their future employment and school opportunities.
I advise you to tell your friend to remove the material from MySpace and to refrain from using the internet until he matures.