If the only reason for the divorce is your mother's Sabbath keeping, then your dad had no Scriptural grounds for the divorce. They are still married in Yahweh's eyes. A divorce by the state cannot nullify the marriage. According to Yeshua, if either remarries, he/she will be an adulterer as well as their new spouse (Matthew 5:32).
Paul, on the other hand, writing his opinion and not the Master's words (1 Corinthians 7:12), says that if an unbelieving spouse wants to depart, they should be free to do so (1 Corinthians 7:15). So, if your dad is not a believer and he wanted a divorce, your mother would be free (no longer bound by that marriage covenant) and could marry another (IMHO). If he is a Christian, then I would think your mother is not free to remarry. Neither is your dad.
If neither one of them has remarried yet, they can still reconcile. This would probably require deep Bible study and prayer by both your father and mother to ascertain Yahweh's will concerning the Sabbath. If your dad is a Christian, then he should be open to this, especially knowing he cannot remarry another because he had no Scriptural grounds for divorce in the first place. I doubt he wants to become an adulterer or cause your mother to become one.
As AbbaLove said, there may be too little information to go on. So these are just my thoughts based on that little info you gave.