I believe my advice is that she needs to be honest. With her husband, with the other man (does he know she is married?) and with herself. Sometimes the comfort of marriage often feels like a loss of love. It doesn't have that same butterfly feeling excitement as the beginning of a relationship does. You don't wonder what they're thinking all the time, you begin taking for granted the things that have become second nature. That comfort is love though, it's mature love.
The woman needs to decide how she intends to spend her life
-Does she still love her husband, if so, marriage/family counselling can still help. If she's going to stay with her husband, she needs to tell him what's going on and distance herself from the other man. Often times the existence of the fantasy person fogs our perception of the good parts of reality
-Does she see a future with the other man? If so, she needs to tell her husband and discuss separation. Even if she doesn't see herself with this man forever, if she doesn't love her husband/doesn't think she can love her husband she can't continue lying to him.
To go a little off topic, does what the husband do for the wife (visiting family, shopping etc) make the wife feel loved? If it were any other person, would the wife feel loved by these actions? Here's why I ask, I read a book called the 5 Love Languages (Gary Chapman) and I know I've mentioned this book in a few posts on here but I truly found it so accurate I can't help but share it. The basic premise is that there are 5 styles to love. Each of us have a "primary" language (some people have multiple or split languages) -this is the language that we feel most loved by and the language we normally express love in. From what you have posted, it seems like the husband is "Acts of Service". He loves the wife so he does things around the house etc that he know will make her happy because that's how he shows love. On the other hand, if the wife did acts of service (washing the car, picking up dinner on the way home etc), he would probably feel very loved. But then things get tricky. Most couples don't speak the same language. So if, the wife is "Words of Affirmation", no matter how many "Acts of Service" the husband or any other man does, she's not going to feel fully loved.
In the beginning love stage of my relationship, words of affirmation just happened. I did all of those cutesy things and my bf felt fully loved because I was speaking his love language (even though I had no idea). But my language is "Quality Time", so as we started to move towards the mature love, I lost touch with my ability to speak words of affirmation. Thus he began to feel less loved; we started becoming distant. When I took the quiz, I knew it was right with my quality time language -it clicked. So I started to guess at what my bf's language would be. NEVER would have guessed it was words of affirmation and I realized how little I speak that. So I've been focusing on trying to step that up and it's been bringing us closer together.
If you found any of that interesting, here's the link to take the quiz. He mentions in the book, that people act differently when their "love tank" is empty. Could the wife's seeking a relationship with the other man be because she doesn't feel loved by her husband?
http://www.5lovelanguages.com/assessments/love/