Sorry to hear of your friends separation. Its never easy regardless of situation.
However, I would like to share with your my story. In my late teens my mother realised that my father's drinking was out of control and basically shifted herself out of the matrimonial bedroom (which I'm not going to think about too hard

) to the conservatory. Once my brother left home, she took over his bedroom and when I left she moved to my old bedroom which she is still in.
Over the years she just came to considered herself to be the paid housekeeper. She never worked out of the home since she had me and felt that she would loose more leaving the home than staying - the struggle to make ends meet and find somewhere to live would be more stress than she wanted as she approached "retirement" years. Dad's wage covered all the household expenses including her overseas holidays once a year or so. She did take up a volunteer job one day a week on a Sunday (one of my Dad's days off) so she wouldn't need to be in the house with him. Saturday's he usually went out for his hobby day.
As my Dad approached retirement (early) she started to get very concerned about having him home all day, every day and was looking for a way out. However, my Dad has since given up drinking with the help of a heart attack about 6 weeks after he retired and AA.
He is now much more reasonable to live with and we have all had to lean to communicate with him again as we basically just ignored him as he wasn't worth talking to. In fact, many people didn't realise he even existed as I never spoke of him.
They still live in the same house but as "flatmates" rather than husband and wife. Dad drives Mum still a little crazy at times but she doesn't feel quite so homicidal towards him since he gave up the booze.