Is it permissible to buy a gift, specifically a gold chain, for a Christian friend?

There is no harm in offering a gift to a non-hostile disbeliever and accepting a gift from him, with the aim of inclining his heart towards Islam. There is also no harm in gifting a non-Muslim a golden ornament for him to benefit from its price or for him to give it to one of his women to wear. However, if the intention is for him to wear it himself, it is not permissible according to the preponderant opinion, because disbelievers are addressed by the ancillary rulings of Sharia. It has been authentically reported that the Prophet (peace be blessings be upon him) gifted 'Umar a silk garment and said: "I did not clothe you with it for you to wear it, but I clothed you with it for you to clothe someone else with it or to sell it." So, 'Umar clothed his half-brother, who was a polytheist, with it. This provides evidence for the permissibility of giving gifts to disbelievers and the permissibility of gifting silk garments to men, but this does not mean that it is permissible for disbelievers to wear silk. As for the Prophet’s (peace and blessings be upon him) prohibition of silk and brocade, and drinking from vessels of gold and silver, and his saying: "These are for them in this world and for you in the Hereafter," its meaning is that disbelievers habitually use them in this world, and it is their luxury that Allah has decreed for them therein, and they have no share in the Hereafter.