I'm grateful for your friends' concern for the environment—but no, they aren't right when they say the Bible isn't concerned about the environment. In fact, of all people, Christians should be the most concerned for the environment.
Why is this? The reason is because Christians know God created the world, and we are only its stewards or trustees. The very first verse of the Bible says, "In the beginning, God created the heavens and the earth" (Genesis 1:1). When we see the world as a gift from God, we will do our best to take care of it and use it wisely, instead of poisoning or destroying it.
We don't worship the earth; instead, we realize that God gave it to us, and we are accountable to Him for how we use it. After creating Adam, the first man, the Bible says, "The Lord God took the man and put him in the Garden of Eden to work it and take care of it" (Genesis 2:15). God didn't tell him to exploit the world or treat it recklessly, but to watch over it and use it wisely. Like a good ruler, we should seek the welfare of everything God entrusts to us—including the creation. The Bible says, "A righteous man cares for the needs of his animal" (Proverbs 12:10).
Yes, we should take care of the environment. But most of all, we should take care of our souls. Is Christ the center of your life, and are you seeking to live for Him every day?