I am so sorry your friend is going through this. She is probably completely overwhelmed by what she feels is her own fault. Falling in love and then marrying someone she really did not know. I am also VERY concerned about her children.
It happens all of the time, but it is not a fault. The best part is she can fix this. She is probably depressed. Part of depression is that you become so tired, you can hardly make day to day decisions, so making a huge decision like leaving or asking for help is way overwhelming.
Let her know that you will always be her best friend. Let her know you are willing to help.
Also see if you can send her this link or print out the list and get it to her, so she can decide what she can do or would like to do.It is a list of signs you are in a abusive relationship.
http://www.recovery-man.com/abusive/abusive_signs.htm
You can use the words. "I miss you." "I am concerned about you." Or if you 2 had a funny thing you used to say to each other include it at the top of the note.. My best friend from college and I can say "Taste Buds" It was a funny incident from college that cracks us up each time and reminds us of our friendship.
I am sending you hope and your friend the truth and strength.