To answer your question, no, there is no rule that says you shouldn’t or can’t have a Bible. In fact, there’s nothing wrong with it at all. As you pointed out, having books from other religions is a great way to learn more about what those other groups believe. If you had a copy of a book of Greek myths or the Talmud or the Bhagavad Vita on your shelf, no one would say anything. And honestly, despite your not being Christian, the Bible can sometimes make for good reading. Sure, it's full of murder and incest and thievery, but there are also stories about the value of peace, love and forgiveness. Those can be useful tools for people of any faith.
A second point to raise – and something you may want to mention if this comes up again – is that this book is a family heirloom. It was your great-grandmother’s. She carried it across an ocean with her. That counts for something, and that’s a powerful symbol of your family and everyone in it. You go ahead and display it wherever you feel like it – it’s a tie to your ancestors, your kin and your hearth.
Now, something else worth addressing is that it sounds like someone has anger issues – and it’s not you, Pagan Guy With Great-Grandma’s Bible. I get the impression that your friend has some serious complaints about Christianity in general, and none of them are your problem. There are plenty of folks in the Pagan community who have had bad experiences with Christianity, or with Christians. None of these things are your fault, and you shouldn’t be expected to jump on the I Hate the Bible Bandwagon just because someone else is on it.