The Bible is an authentic piece of ancient literature. The Book of Morman is not. There's all kinds of things you could do to point this out
You could ask them why the Book of Mormon is written in Jamesian 16th and 17th century English, when that is not the era Joseph Smith lived in. Was Joseph Smith deliberately trying to give the book a "Biblical" flavor by putting it into the same English as the King James version? If he was, then his unfamiliarity with Greek would have tripped him up:
For example, you could ask them how the word "Synagogue" found it's way into the Book of Mormon, since the word wasn't coined until Alexander conquered the holy land and the Jews began speaking Greek for normal day to day activity. The BOM was allegedly written before this happened.
You could ask them how a Greek expression that only John used, Amen Amen lego soi - "Verily, verily I say unto thee" found it's way into the BOM (Again in Jamesian English)
You could thumb through the BOM and look at some of the pictures. The prophet Abinidai standing before King Noah for example. The picture, and more importantly the narrative upon which it is drawn from depicts an advanced culture on par with the Roman Empire.
You could observe, "Cultures at that level produce durable objects. Masonry ramparts, walls, arenas, temples and columns. Kiln, fired pottery, tile and mosaic work. Steel swords, armor, spear and arrowheads. Gold and silver jewelry, coins, statues, temple ornaments, etc. These things last many thousands of years. You can go to any good museam and see Egyptian artifacts that were already many centuries old at the time of Christ."
You could ask, "Where is the evidence that any of what the BOM describes actually took place? Where are these type of artifacts?
Even if the Bible were purely a work of fiction, it would still have historical value. There is no doubt in anyone's mind that the Roman, Greek and Egyptian empires existed because the durable objects they produced are still with us today. The pyramids still stand. Greek and Roman temples still stand. You could tour the holy land and see the wailing wall, the ruins of the house of Caiphas, Hezekiah's tunnel, the pool of Siloam, the mount of Olives, etc. Why should anyone accept the BOM as an historical work?"
I've not even scratched the surface here. This kind of stuff goes on and on. DNA evidence refutes the Lamanite theory. The horse was unknown on the American continent at the time the events described in the BOM allegedly took place. Then there are the Kinderhook plates, substantial differences between Joseph Smith's handwritten account of his first vision, and official church history, etc.