She writes:

I work out of Shreveport, Louisiana, just right along the Bible belt, where Catholics are a minority among the Christian community. In my workplace, it is clear that I am Catholic (saints calendar, purgatory book, and a rosary right on my desk). It's my mini way of evangelizing due to the fact that God did not gift me with an eloquent tongue on fire.

I had a Baptist coworker (we talk a lot on theology and prayer openly at work) who came up to me today apologizing for his disregard in helping me at work yesterday. I told him to think nothing of it, which turned into a friendly conversation about what we did after work. I told him, since I don't have internet at home and there was nothing on television, I was reading a book. He asked which book, and I told him "Hell and How To Avoid Hell" by Fr. Schouppe, S.J. It used to have the imprimatur until 1983, when books of a general religious nature no longer required it. I read this a long time ago, but felt like re-reading it again.

The conversation moved to the topic of the location of Hell, and he insists that it's in the center of the earth.

I was utterly confused and asked him to explain to me the theology behind that assertion. He just said "research it" and walked away. I went about my work for 10 more minutes and he came back asking if I researched it. I told him that it sounded unbiblical and asked him again to prove it. He said the same thing ("research it") and walks away.

I checked the catechism for it (1033-1037) and did find that the Catholic church did not teach that idea. I do know the phrase from JPII about how it is more of a state of being than a place.

The only thing I know for sure is that he will come back to me and try to prove to me that hell is located in the center of the earth, and I do not wholeheartedly agree with it unless there's some good theological basis. Any help?

I appreciate your time in explaining this :]

It's his job to prove his theories, not your job to disprove them. So the proper response to such claims is "Documentation please?" One can, I suppose, cobble together a few Bible verses that speak of the place of the dead as under the earth (that is where they are buried, after all) and something from Phillippians about how those "under the earth" will bow before Christ along with the rest of creation.  But it's a mighty thin thread of reasoning to get  from there to the confident certainty that hell is in the center of the earth.  You are right to stick close to the Church's teaching--both what it says and what it refrains from saying.  In the end, the main thing about Hell is that we are to avoid it. 

Not that it cannot be held as a pious opinion.  St. Thomas, for instance, thought that hell was beneath the earth (though he distinguished that from the center of the earth).  And given that the damned as well as the saved are to be raised and will experience punishment in their bodies as well as their souls (just as the righteous will experience beatitude in body and soul), who's to say he's wrong?  But as far as I know, the Church tends to leave such details in the realm of pious speculation.