Does custody automatically move to the father if the mother remarries in the UAE?

No — custody does not move automatically, and your colleague is repeating a common simplification. Under UAE personal status law, the mother's remarriage is a ground the father can raise, and historically it could lead to her losing custody, but it has always required the father to actually apply to the court, and the judge decides. The current federal personal status framework puts the best interests of the child at the centre of every custody question, which gives judges real discretion: they look at who has been the child's day-to-day carer, the stability of each household, the child's age and schooling, the circumstances of the new marriage, and whether moving the child would genuinely serve the child rather than simply penalise the mother. Courts have kept children with remarried mothers where that was clearly better for the child, and have transferred custody where it was not. So if your ex-wife remarries, you would have a recognised basis to ask the court to reconsider the arrangement — nothing more, nothing less, and certainly nothing automatic. If this is becoming a live issue rather than a hypothetical, speaking with a UAE family lawyer first will give you a realistic view of how a judge would weigh your specific facts.