Well, there is only one God, and He cannot teach His people two different things, for He is infinitely coherent. Clearly, the oneness of God implies that to worship any other God would be to greatly offend Him.
In our case, for instance, we profess that the Hebraism and Islam worship the same God we worship, though in different ways.
With regards to other religions, from the Catechism (
here):
842 The Church’s bond with non-Christian religions is in the first place the common origin and end of the human race:
Code:
All nations form but one community. This is so because all stem from the one stock which God created to people the entire earth, and also because all share a common destiny, namely God. His providence, evident goodness, and saving designs extend to all against the day when the elect are gathered together in the holy city. . .331
843 The Catholic Church recognizes in other religions that search, among shadows and images, for the God who is unknown yet near since he gives life and breath and all things and wants all men to be saved. Thus, the Church considers all goodness and truth found in these religions as "a preparation for the Gospel and given by him who enlightens all men that they may at length have life."332
844 In their religious behavior, however, men also display the limits and errors that disfigure the image of God in them
Much more could be (and has been) said. However, ultimately there is only one God, and it is irrational to conceive that He would allow His creation to worship as God anything other than Himself
Don’t be too upset, though: if he talks a lot about religion, it means that there are strong motions in his soul and that he is shaken in the search for God; not all have the wisdom to react in an open-minded way: some chose to react by becoming obstinate and rejecting the revealed truth. Be very patient, as Christ would be, and do find out more about the Sacred Tradition (as you are for instance doing here, but also by reading the Catechism and some good books on Apologetics) so that you may exercise the teaching authority that belongs only to the Church, becoming in your little way a “new evangelizer”
Remember, though, that Christianity is characterized by charity and forgiveness. Love is patient and kind, especially towards those who seem to ignore or offend God: for they obviously “do not know what they are doing”.
It may help, for instance, to mention that “religion” is how we relate to the supernatural. If I believe in one, many, or no Gods, that is my religion. Some speak even of a “secular religion”. Often, people use “religion” as in “community of faithful” or “doctrine”, etc. That is an erroneous statement. Nobody can say that he has no religion, for we are all naturally confronted with the supernatural world, as a result of our soul, being spiritual and longing for God, in whose image and likeness she is created.
So back to your original question, I guess we could say that the doctrine is not fully developed or not fully correct. And since you and me do not know everything, you can always refer him to the approximately twenty centuries of Sacred Tradition, which answers basically any question if people just were humble enough to search for answers…
I didn’t remember ever learning about this possibility (although I must have as I was a religious studies major in college?) It really made my mind stop for a second. I responded “OH, well my faith teaches we all have original sin” and left it there.