Joe, it seems to me that you are asking the wrong people for evidence. If it is their position that this soul exists, it is incumbent upon them to prove that it exists. Even if we didn't have any evidence that a mind or someone's personality is dependent on a physical mind, they would still need to prove that a soul exists before they could ever even begin to hypothesize what it does.

What evidence do we have that memory, personality, or a mind is dependent on a physical brain? Well, first of all we have absolutely no evidence of these things existing without one! After that it's fairly simple, we have evidence of people suffering from amnesia after serious brain trauma. Someone could take drugs or ingest alcohol which has an effect on the physical processes of their brain and can lead to a change in inhibitions, memory loss, and a change in personality. There are many, many case studies of people experiencing vast personality changes after suffering from a severe brain trauma. If these weren't a byproduct of a physical brain, then why are the affected by physical trauma, prodding, or chemical reactions?

I would also ask, if there is a soul, then where is the mechanism in the brain for where it interacts with the brain. It would seem that they think that the soul is immaterial and a non-physical force, so how is a physical object (the brain) impacted by the soul? Where does the soul exist? If it is an immaterial force, then how do we detect that there is one, and if we can detect it doesn't that mean that it can be detected and tested? Then it's no longer a supernatural force, it's just a part of nature.

Put it back on them though. They are the ones claiming that something exists for which they cannot demonstrate. They are the ones claiming that this undetectable force is known to them and that it defies everything that we have come to know about how the brain is structured and works. Put the onus back on them, it is not incumbent upon you to prove that their supernatural nonsense doesn't exist.

Also look up the studies regarding split-brain patients and how each half of the brain is different. One side may believe in a divine being and the other may be an atheist. You should ask them which half goes to heaven and which half goes to hell.