Thank you, XXXXX XXXXX requesting me for new questions. It is greatly appreciated. Once again, I apologize for the brief delay. Thank you for your patience.
Unfortunately, out-of-court agreements regarding custody/visitation are not enforceable. However, if the previous arrangement was court approved and if her ex violated it, that would provide her with a basis to motion for contempt. Further, if the previous child support was court ordered, that would provide a basis for contempt motions as well. It's also worth noting that, if her husband wasted marital assets on his affair and if she can show the court as much, the court can reduce his share of marital assets due to the waste.
As for the custody matter, it really depends on the details. Custody matters are highly detail-oriented. As we discussed in previous questions, courts really do look at everything to see what's good for the child. Her move could work against her while his affiar could work against him, but those are only factors that the court look will look at. Of course, as we discussed, a legitimate move for legitimate reasons (such as for business or family reasons) is often understood by the court, though the court will still look at its effect on the child. If her husband is cohabiting outside of wedlock, that can work against him as well. The court can even enter an order (with regard to custody or visitation) that no overnight guests are permitted when the child is present. Mediation is often part of the divorce process, but that's not where it necessarily ends. Mediation is simply there to try to help to avoid unnecessary and costly litigation - in all honesty, it's a very good idea in many instances, because an amicable (and reasonable) settlement without a trial costs everyone less time, effort, money, and stress. However, if they mediate and fail to settle, they can still go to trial.

Please remember that your rating is for the quality of my work and the information I provide. These ratings are not related to the quality of the law (rest assured, I agree that the law is often lacking). I would be very appreciative if you would rate this answer as "Great Service." Low ratings reflect poorly on the quality of my work. If you have additional questions, please ask them using the "Continue Conversation" feature. Thank you.