For federal tax purposes only, the custodial parent is determined by the IRS rules only. Per their rules, the custodial parent is the parent with whom the child lived for the greater number of nights during the year. The other parent is the noncustodial parent. You are neither parent. If the child live with the mother in 2017 for more nights than lived with the father, then the mother is the custodial parent and can claim the child as her dependent.
