I would caution you to tread very carefully, nobody ever knows what really goes on within a marriage, if you get involved in trouble between them you may inadvertently make matters worse.

I think the best you can do is be good company for your friend, in that way you are helping her be happier in the time she is with you. Only she can decide what to do about her marriage, you might want to reassure her you are there for her whatever she chooses to do.