The person who shared the update on reddit is the person who, as a friend of Sarah's, has been posting relentlessly from the start (first post?) about her disappearance and for her safe return.
The poster on here who shared that info identified herself early on as a personal friend of Sarah's and also a friend of the reddit poster.
They have chosen to share the outcome, and as such, and being privy to the information, will also likely be privy to the knowledge of who already has been informed.
That is their information to share if they wish, and they should not be berated or attacked for doing so.
My late husband's passing unavoidably became known to some people who knew him well and cared for him via social media, purely because I did not have the strength to inform everyone who knew us personally myself. I've no doubt that this was a terrible shock, however, I and they understood how this came about, and did not hold it against anyone.