M.P.
Please do not blame yourself! You said they sent your son a gift just before this happened. She knew where you were. This did not happen because she needed help and you weren't available. Tell yourself that every time you start to feel responsible. You are not responsible.
Yes, you'll grieve but do not allow yourself to blame yourself. Perhaps blaming yourself is a way of denying the reality of the situation which is the first stage in grief. In reality, there is nothing you could've done.
I suggest that you're expecting more from yourself than you have a right to expect. They moved away. You were their friend but you didn't live near them. Let go of this way of thinking. You're only making grief more painful for yourself and grief is already painful.
Here is an article about the stages of grief. http://grief.com/the-five-stages-of-grief/