One misunderstanding some people have about Christianity, Judaism, and the Bible is that they are not the oldest religions and others predate them. They will cite some eastern religions as being older. When it comes to ancient religions, we can date their origins by surviving records, secular references, or their own claims as to when they started.
The problem with ancient religions is that they are ancient. Thousands of years ago they did not have computers or even printing presses. They wrote on parchments and other materials that would not last forever. Some faded or fell apart. Some were lost in fires, or attacks. Some were buried over time. Only a fraction of what might have existed survived to our time. This makes precise dating all but impossible.
In the case of the Bible, its story begins with creation itself. Genesis tells us of the creation of the universe and of humanity. God walked in the Garden and talked with Adam and Eve. They were the first believers. God appeared to their children and grandchildren and so on. Eventually the nation of Israel was formed from this line of believers. Moses, wrote the first 5 books of the Bible, before the nation of Israel fully existed. He was given the history by divine inspiration. Since no history predates creation, and since the story of creation is the beginning of the Bible, the Bible and belief in the God of the Bible, predates all manmade religions. While we see Judaism and Christianity as separate religions, the New Testament teaches that Christianity is the completion of Judaism. Judaism was meant to lead to Christianity with the coming of Jesus Christ the Messiah long-awaited by the Jews. Christian history begins with Genesis just as Judaism does. Based on that, I would date Christianity and Judaism as the oldest religions on earth.
When people claim these two religions borrowed stories from older religions (like the flood, resurrection, etc) they are not allowing for the possibility that Judaism and Christianity might be older and these other religions borrowed from them. Naturally, some people will debate this but I will never concede the point that any religion predates Judea-Christian history. Besides, being first does not necessarily make you right. When friends have tried to explain certain eastern traditions to me, and I have responded with Scripture, they often say “Well this stuff was around long before the Bible.” While it may have been written before the Bible, that does not mean it occurred before the Bible nor that it’s correct.
Secular scholars date Hinduism (or at least the root of it) as the oldest religion dating from 15 BC to 5 BC. They claim Judaism dates from 9 BC to 5 BC. Some claim Judaism is rooted in Atenism which is an old Egyptian monotheistic religion. The only similarity between Atenism and Judaism is that both were monotheistic. Atenism was based on the Sun god. That Judaism is monotheistic in no way proves it had any connection to Atenism. According to the Bible, belief in a single God goes back to Adam and Eve thousands of years before Atenism. Dating a religion upon the earliest writings or artifacts ignores the fact that so much of antiquity has been lost to the sands of time. Did Adam and Eve write anything down? Did Abraham? We don’t know but nothing has survived to us but that should not be surprising given the amount of time that has passed and the materials they used for writing. Note too that Moses was believed to live around 15 BC thus at the earliest suggested date for the start of Hinduism and Moses is far from the first person mentioned in the Bible.
Ultimately, this is an argument you won’t win as your friend may not accept your statements about the Bible being older. I would rather attempt to get them to admit that what we know is only known from surviving fragments or artifacts. It’s possible there were religions before Hinduism that simply did not survive. Secular science believes life began in Africa requiring many years before any descendants would have made it to the east. Who’s to say they did not have beliefs that predated Hinduism? Perhaps if they will admit we can’t be 100% certain then they will have to admit that you could be right about the Bible.