If your friend is getting into the so-called “historical Jesus,” I’d recommend reading some of the scholars who actually use the historical-critical method in a responsible way, e.g., N.T. Wright comes to mind. Pope Benedict has also written some very scholarly works, e.g., Behold the Pierced One or even his latest book.
It’s tough to provide a blanket refutation since this is a very broad topic. In general, I’d keep the following in mind: Not that much time elapsed between Jesus and Paul, Paul shows clear evidence of holding to a pre-existing tradition, none of the apostles acts as if Paul is doing something completely new. There are really a lot of points you could make: Jesus wouldn’t have been crucified for trying to reform the temple, the very early Christians believed in the Resurrection, worshiped Jesus as God, etc. and had to get this idea from somewhere.
Good luck and keep us posted!
