If this happened within the game, the best course of action is to report it ingame at the time that it is said. This captures the text and sends it to Jagex's Player Support team for review. And yes, that would be something that Jagex would want reported to them, as they want players to feel save when ingame.
Its a bit too late to report that now, as the ingame report system only captures the last 60 seconds of chat, but definitely do report in the future if this happens to occur again. Jagex typically does not accept screenshots as evidence, as they can be edited to depict a situation that is different from what actually happened.
Finally, the first post was hidden as it named the player in question, as naming other players in a negative light is something best avoided when using the forums. I've left the thread open for a bit though in case you had any additional questions.