Welcome, Adfinem. I echo the others' suggestions. Please do forward that message to the authorities. I, too, believe it's important evidence.
Also, the content of the message gives me the impression SS did indeed enjoy a certain amount of power and weight in the relationship. She "spoke" for him, which I find rather unusual in the context of social networking. Especially since you shared a friendship with NS in high school, and you weren't a mutual friend. She kinda "took the reins" on that one.
Further, she's referring to herself and NS as "us" and "we" and emphasizing their "happy unity." That seems to demonstrate she's going WAAAAAY out of her way to make that clear.
Please don't take offense to my speculation here, and forgive me if I'm way off base, but I haven't read anything that points to your gender. If you're female, that would give SS more motive to step in and "establish" herself, especially if she felt threatened in any way (and from what we've learned about her from her own acquaintances, she was VERY jealous and possessive). If you are male, I would think she'd be less concerned. BUT...guess we can't know for sure.
In any case, the TIMING of this message from SS is what's so bizarre. She seems to be going about her social life as if nothing at all is out of the ordinary. She continues with her "happy unity" and her social dramas, and yet, in all likelihood, her four-year-old son is dead or dying. ????
Thanks so much for sharing this information with us, Adfinem!