My two cents:
For the most part I believe these pre-Christian, non-Judeo religions are human attempts to reach the divine which humanity, before Christ, found itself alienated from. Also, there is the possibility that some of these faiths were introduced by the devil. I know the psalms speak of this, as well as Milton
) . I've had these "human striving" religions explained to me in the "onion dome" analogy. If you take a circle, this circle can represent all human potential and possibility. The ability to be cruel, arrogant, prideful, lusty, etc. all falling somewhere in the bottom portion of this cirlce. Other human qualities such as kindness, a certain measure of abstinence from sinful behaviors, selflessness, charity, llove, etc. all fall somewhere in the topmost section of this cirlcle. However, as was said, this circle merely represents human possibility with perhaps some of God's will working from the outside
. Now picture a dome with a nice point attached to the top of that cirlce. This dome is Christ. With Christ , after he put on humankind, and we, through baptism are grafted onto his body, can transcend this circle. Now the human qualities can be infused with the divine and lifted outside of this circle to what, with Christianity, is this onion dome of sorts. Being brought outside this circle to the dome is the salvation of Christ. Without Christ though, no matter how good and pious we are, we can't reach the peak of that dome and therefore have salvation. Does that make any sense?
Also, some elements of these pagan religions, whether in stories of the Virgin Birth in Greek mythology, or perhaps the story of Krishna coming to Earth in the Bhagavada-Gita, or maybe even some aspects of Norse mythology which I don't know about, may all serve as prototypes for the real event of man's salvation through Christ. Some modern biblical scholars, and even a few who claim to be "Christian", make the same claims as your friend- that the Christian "stories" were all influenced by the mythology of the past. However, if Christ really is the redeemer of the whole world, why could he not redeem and sanctify the mythology of the past? Or, perhaps these myths, being products of mans' desire to come into contact with the divine(i.e. salvation) are just more testaments to the failure of a mankind without Christ, represented in pale images and stories in preparation for Christ. The myths are kind of like the shadows on the walls of Plato's cave. Perhaps mankind collectively and instinctively knew that real salvation would come about through certain means- virgin birth(greeks), God coming to Earth(the Vedas), etc., and these events were worked out into stories still within that realm of the circle, not yet topped by the dome of salvation.
Hope there's some sense to all of what I just wrote.