When I told my friend who is a Hindu that idolatry is wrong, she said that as we Christians take the cross as our representation we take a idol and worship it. How am I supposed to answer such a question? Although its not the cross itself that we worship, but it represents faith in what He did on the cross for us.
We need to establish a definition of idolatry that you and your friend can agree on. Otherwise we are arguing about “apples versus oranges,” which is not a useful discussion. Let me propose a definition of idolatry. Idolatry is when a human being worships an object or a “thing” rather than worshipping the Creator or the supernal reality. For a Hindu, this might be to worship using a statue of Ganesh or to pray toward a temple or to rely on a ceremony. For a Christian, it could be going to a shrine to pray at a statue of Mary or of a “saint.” Some people use crosses as physical objects through which to pray to they go to a shrine to touch something, hoping to get healing. This would be idolatry.
What you could say to your friend is that it is a common human frailty/sin to rely on physical objects for worship or to focus on created things which are seen rather than on the abstract “God” or “Brahman.” Both Christianity and Hinduism have included some rather blatant idolatry in their forms of worship. This is particularly obvious in the versions of Christianity known as Roman Catholicism and Orthodox Christianity, but it is not limited to these. Hindu worship in India (as opposed to the kinds of Hinduism generally practiced in the West) is saturated with idolatry. However, to accuse all Hindus of idolatry is not fair. Neither is it fair to say to Christians that the use by some of the cross in Christianity means all Christians are idolaters.
With this ground for discussion, I believe you can show your friend many passages against idolatry, both in the Old and the New Testament. Isaiah went on at length about this. The Ten Commandments are helpful here. Then you can tell her that the early church completely rejected the idolatry which had crept into Judaism (although even the Judaism of the first century was actually much freer of idolatry than ancient Judaism). The cross was not used at all in Christianity as a symbol for at least two hundred years. Only later did idolatrous practices enter into Christianity.
You can share your conviction that, as a Christian, you pray to God directly and do not use physical objects of worship. You reject the idolatry of Mary worship or the use of rosaries or crosses as items of adoration. Hopefully, as a Christian you do not use a cross as a means of worship. You should instruct her gently, being careful to bear in mind that some Hindus are as vehement against idolatry as we are, even if this is a minority position in India. I believe that if you use humility and wisdom you will be able to help your Hindu friend agree that idolatry is problematic in any religion as it can take focus away from God and turn it toward the physical things.