Wow, i'm sorry for your friend... This is huge. Unfortunately i have limited knowledge of the U.S police investigation procedures and criminal law, but if you have screenshotted the status update, you can certainly put it forward as evidence for the police, as a concerned citizen. I'd however advise against any kind of public witch hunting towards his ex or even discussing it before you do so, as that might be considered slander. If/when the police has investigated the matter, then you're safer discussing this possibility.
If the police refuses outright to pursue this avenue of investigation, maybe you can take it up straight to the D.A? I don't know if that's a possibility in the U.S, but here in Brazil you can present evidence of a crime straight to our equivalent of the D.A, which can then in turn pressure the police to investigate the crime, or outright prosecute based on the evidence you have provided, as police involvement isn't technically necessary for prosecution of a crime, only the evidence of a crime is necessary.
IANAL in the U.S though, so someone with more experience and knowledge in the matter (especially with all the different state laws you guys got over there), is your better bet for discussing it.