I have to agree with the others that this is something that you, as a friend, are going to have to let both she and therapy handle.
Some people go to therapy very deeply hurt, but they are willing to do the work as it comes. Admitting where we, ourselves, played a role in our own unhappiness is very hard and humbling, but it allows us to acknowledge our mistakes, move on, and make better choices.
If it were my friend, I would be wondering what's behind both her choice to maintain the 'mediocre' status of her marriage (because that's something she can address constructively in the here and now) and her inability to let go of the past. Did she get therapy between being married to her ex and getting remarried? Sometimes, we repeat mistakes-- or make different mistakes-- if we don't have a chance to look at our past in a reflective way.
Only your friend can help herself. Maybe she needs a different counselor. Maybe she isn't ready, yet, to change things. Some people can go to counseling, hear what the counselor is saying, but not be ready to address deeper issues. Some people are afraid of doing this, afraid to disturb the status quo in their own lives. I don't know if this is what's holding your friend back, but after 30 years, one would hope that she'd moved on enough not to care any more.
Is she the kind of person who is always judging her life against the lives of others? Until one is emotionally secure and comfortable with themselves, this tendency to use others as a yardstick can undermine happiness.
Here's the hard truth, J., and I am sorry to say it-- until your friend really wants to be healed, until she is willing to forgive him and let it go, until she is willing to address the problems in her life in the present, past hurts will continue to be a burden for her. Therapy can be a long, long journey. Some people are in therapy for years before they really feel released from past pain. And some people cling to that pain and use it as an excuse for not living their life fully. I don't know where your friend stands in this regard, but please know, it is not your responsibility to help her do this. I say this as both someone who was once very troubled and burdened by the past, and as someone who has tried to help a few people who weren't ready to let go yet. Being a victim of the past was a part of their identity they weren't ready to give up at the time. It's a tremendous drain on a friendship and people cannot do the deeper, harder work until they are ready to, period. So, you can be a friend to her, but know that this responsibility doesn't fall on you-- this is something you can stand by her through, but nothing that you can fix. You sound like a very caring friend-- I hope your friend can move forward happily sooner than later. And I really agreed with others responses-- if it's a seasonal thing, that's one thing, but if she's focused on it a lot, that's a whole other thing indeed.