Ymd - thank you for the information.
It would seem that biblical counsel and information IS relevant, so let's address the first crucial point first.
I know she knows that God hates divorce but she is not willing to open up to me as to what sign she got.
This is dishonesty. It is also "unacceptable" since she has stated that she has been told by God that divorce is "okay" for her.
So let's operate for now on the assumption that she IS a born again believer who has been "ensnared" by sin and is fighting against her flesh the same way Paul that stated that he fought against his flesh. If you'd like that particular biblical reference, let me know and I'll provide it to you.
The question you seem to be asking, however, is what can YOU do to help this situation.
The answer is perhaps "uncomfortable," but it necessary nonetheless. You MUST "press her" for precisely WHAT "sign from God" she thinks she received that would make God go against Himself. You need to know what she is thinking before you can address those "thoughts."
Also, if the person she is receiving advice from (that she respects) is a believer, then that person cannot also be an "enabler." It is also possible that that person does not have experience in dealing with adultery and recovery of a marriage that has suffered from infidelity, so how willing do you think your daughter would be to advice from someone who is both a believer and who has "gone through the fire" of infidelity?
If she is willing, I can provide you with some things that would be helpful, but she will need to read them and discuss them.
In addition, it has been my experience that not a lot of Pastors, and even fewer Deacons, have any practical counseling training for the specific needs of infidelity. Most Pastors only have general "counseling" as part of their Seminary coursework and can be thought of as (in medical terminology) "General Practice" people. They are very good at the "basics" but what is needed for severe trauma and illness is a "Specialist." Most good GP's will refer a patient to a specialist when their own expertise level is "exceeded," simply because not everyone can know everything about everything.
You daughter and son-in-law NEED to be in JOINT marital counseling with a trained counselor who is committed to both the Word of God and to saving marriages that have been devastated by infidelity.
If you would like to know if there some counselors of that type in your area I can also provide you a link to a National group that focus' on Biblical Counseling.
So, again, let's start with her "feeling" that God has told her that she can, or should, get a divorce. What it seems she is believing is the very same lie that Eve believed; "God didn't REALLY mean what He said...."
God bless.