All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
You indeed did wrong by helping your friend find the cross because the belief of the Christians in the cross and their worship of it are manifestations of their doctrinal deviation and association of idols with Allaah, The Almighty. It is impermissible for the Muslim to approve of the deviations of the disbelievers or help them carry out acts of disbelief. Muslim scholars underlined that it is prohibited to offer the cross as a gift to Christians or sell it to them because this constitutes approval of their disbelief and their worship of it. It also constitutes helping them in worshiping other than Allaah.
Shaykhul-Islam Ibn Taymiyyah was once asked, “A Muslim tailor sewed a silk sash bearing a gold cross for Christians; does he bear a sin for sewing it? Are the fees that he received lawful?”
“Yes, if a man helps another to disobey Allaah, then he bears a sin for it, because this is cooperation in sin and transgression. This is why the Prophet, sallallaahu ‘alayhi wa sallam, cursed Khamr (intoxicants – alcohol, wine, etc.), the one who drinks it, the one who pours it for others, the one who sells it, the one who buys it, the one who makes it, the one whom it is made for, the one who carries it, the one whom it is carried to, and the one who consumes its price. Most of these people, namely the ones who make, pour, serve, and carry it, help the sinner who consumes Khamr in committing that sin. This is why it is deemed forbidden for the Muslim to sell weapons to those who would use them in prohibited fighting such as fighting Muslims or fighting during times of Fitnah (tribulation). If such action constitutes cooperation in committing sin, then how grave is it to help others perform rituals of disbelief! It is prohibited for the Muslim to make or sell the cross either for free or in return for fees. It is also prohibited for the Muslim to make or sell idols. Also, the Prophet, sallallaahu ‘alayhi wa sallam, said, 'Allaah has prohibited the trade of alcohol, dead animals, pigs and idols.' Moreover, it has been authentically reported that he, sallallaahu ‘alayhi wa sallam, cursed the image-makers. He, sallallaahu ‘alayhi wa sallam, did not leave an image without obliterating it. Therefore, the one who makes the cross is subjected to the curse of Allaah, The Exalted, and His Messenger, sallallaahu ‘alayhi wa sallam ...”
Allaah knows best.