I think if he wants to stay in touch with his best friend I completely get that. I will not be having any contact with her (his best friend's wife) and I'm going to talk with him about the toxicity of their relationship (playing the victim, blaming others). I really think while we're working on things there needs to be no contact period.
If she needs help moving etc later on - then I'm willing to do that but only as long as we do it together.
I really think he got a taste last week of her unstableness and maybe had a little bit of a wake up call about how much she was actually "helping" him.