From what I understood, that is not what happened at all.
What I read from the family was that biomom had most of the custody for about 2 years until she had kidney failure and had to get treatment in Canada. Her best friend still lived near her ex, and she asked the SM to move in as a caretaker for Kyron while the biodad worked. They were already divorced, and there is not one statement that says it was not amicable, nor that biomom was stabbed in the back- she ASKED her friend to do it, so she obviously did not care, and after a couple of years, she remarried, so why would she care?
I don't like friends who steal people's spouses any more than the next person, but the family has pretty much said this is NOT the case. I set my ex-husband up with my best friend years after our divorce. I knew they'd get along. Not everyone is mean and spiteful about their exes.
Until the family themselves say unkind things about each other, I am not going to let stereotyped ideas about families, or how people should react under grief influence my ponderance of the facts.