My response:
From a legal standpoint, your friend's boyfriend is absolutely WRONG. California hasn't been a Common Law marriage State in over 100 years; and, separation has nothing to do with divorce in California. You could be separated for 50 years. But, without that divorce decree in your hands, you'll always be married.
Now, from the standpoint of him wanting to get your girlfriend in the sack, his story is absolutely correct - - hell, I'd use it if it meant "a good time".
Moral of the Story: Men will use any excuse they can to get what they want from a woman. Men can be scum. But, it's perfectly normal to do what he's doing. Men are also "hunter / gatherers" and what he's doing is merely instinctual, caveman stuff.
IAAL