Yoga is perfectly OK if you're a Christian. Yoga is a form of mind over body control, originating in India in a non-Christian culture, but it is by no means contrary to Christianity. Rather the reverse. Certain Christian saints like St Augstine of Hippo carried out a mental self-discipline similar to Yoga (though they wouldn't call it that). I think your friend is worried that you might be drawn to Hinduism. She is being afraid, ignorant and narrow minded (no offence to her). Do what is right for you, and do not worry about other people's talk about the wrath of God. God will reach you if you seek Him.