The situations when the man moves in with the woman's best friend are fairly common. Not rampant, but do happen from time to time. You need to realize that it is twice bad for your mother - she lost her husband AND her friend and she feels betrays be her husband AND her friend. So she needs therapy. Can you suggest it to her? She probably also needs antidepressants - that would be my guess.

But definitely nothing is your fault and there is nothing that you personally can do.