I should have thought it was up to them. Jesus is a "Moslem" prophet, so if they want to celebrate his birthday, I wouldnt see anything wrong with that. That is, assuming he had a literal or physical body.There is a "Christian" perspective whereby some Christians are of the opinion that he could not have had a normal body since he was able to walk on water, and disappear through walls etc.
Another aspect is that "Jesus" was a physical man, and "Christ" was a spirit which was the essence of God blown into his mother Mary simultaneously coinciding with his conception.
Again another aspect is that Christ is a spirit within us all, and salvation comes about by symbolically crucifying ones corporal nature (dying) and being reborn into the spirit of Christ (within).
Ancients of the Mystery Schools, from which Christianity is thought to derive in some measure believed that the 25th of December was the birthday of Osiris-Dionysus, and the Christians "stole" the date and used it as the birth of Jesus. Of course, belief in Osiris goes back to the time of ancient Egypt.
Apart from religious reasons, in the UK and many parts of the world, this period of the year is a public or state holiday.
Whether one wishes to have a good time and celebratge for the sake of celebration or for religious reasons, its entirely up to the individual, I would have thought. Charlie.