Unless said friend knows too much about you and it's more of a liability to have said friend no longer a friend!
Exactly what I've been thinking. But if someone has something over you, I'm wondering why engage them in the ultimate crime, which would bond you to them for life?
It's just so disturbing, and such a contrast, that the victim was so well respected, and very concerned with her patients' health and well being, and no matter how you look at it, her husband brought darkness and death to the family.