In my experience, it's best not to get involved unless you know of abuse or neglect and in that case, a call to your local family services would be the best place to start.
If it is a disagreement between husband and wife about their son's sleeping location, I would not get involved.
You might say to your daughter, "I really am sorry to hear about your struggles but I am not equiped to help in any way other than being a good listener. If you need help, please talk it out with your husbamd or get counseling through local churches or professionals." Then politely change the subject.
If you get involved and they reconcile or even if they do not, you could be blamed for the results.
It sounds to me that this couple is using their son as ammunition for their own arguments and that is not fair to the child and caustic to their reconcilliation. This is a problem for your daughter to solve and she is capable so let her.
Luise gave you very great advise and is the wisest among us. Read it again to see if her words apply to your current situation?