This is a great question to be asked! Without knowing exactly what your co-worker is thinking, I can’t give a specific answer, but let me suggest that there are two things you could do.
Firstly, you could try asking your co-worker what they believe about God and how they know about God. If they believe in God but not the Bible, then you want to get an idea of what their ideas about God are and where they are getting those ideas from. They may, for example, have many ideas about God that do come from the Bible, but feel no need to read or know the Bible itself.
Secondly, you will want to explain what the Bible is and why it is important. A great place to start thinking about this is Heb 1:1-2, “In the past God spoke to our forefathers through the prophets at many times and in various ways, but in these last days he has spoken to us by his Son”. The Bible is God speaking to us and revealing himself and how he may be known, most of all in the person of Jesus (2 Tim 3:16-17 is another great passage to look at). The importance of the Bible then, is that without the Bible we do not in fact know God. Jesus speaks about himself in this way, in John 14:6, when he says “I am the way and the truth and the life. No one comes to the Father except through me.” It is only through Jesus that we know God, and only in the Bible that we meet with Jesus.
If the issue your co-worker is dealing with is the reliability of the Bible, then perhaps this other article will help:
How do you know the bible is real and true?