What did CS Lewis think didn't reflect God?

I think your friend either misunderstands you or Lewis. You will find a nice essay by David Allred, comparing C.S. Lewis and Plato, at "Into the Wardrobe".

Things in themselves

It is perfectly Christian to reflect that all things in themselves are good. Indeed, Paul asserted the idea just as clearly: "I am convinced, being fully persuaded in the Lord Jesus, that nothing is unclean in itself." (Romans 14)

Thoughts in themselves

Now, you do muddy the waters a bit when you bring creative works into the picture. By "creative works", I means things to which humans have added meaning, such as "Snow White and the Seven Dwarves" or "The Jerry Springer Show". The things in themselves, certainly, are not unclean. But it is difficult for us to dissociate the things themselves from the meaning we ascribe to them or the meaning intended by them by their authors. And there we have reason for reservation.

Jesus said, "Whoever shall speak a word against the Son of Man, it shall be forgiven him; but whoever shall speak against the Holy Spirit, it shall not be forgiven him." I bring up this quote only to highlight this fine distinction. In light of Paul, we would say that merely uttering the sounds of a blaspheme against the Holy Spirit is not what Jesus is warning against — otherwise, quoting someone else blaspheming the Holy Spirit would be equally condemnatory, which would place the verbalization itself on the order of an incantation. Of course, this isn't the point. As Jesus said elsewhere, "Out of the heart come evil thoughts, [etc.].... These are what make a man 'unclean'."

The subtlest evil thoughts contain a good deal of truth (which is why they are so deceptive); and it is to these fragments of truth we can cling, and by which we can grow, if we are vigilant, humble, prayerful, and earnest. This, in fact, is how any of us grow at all: we swim submerged in a sea of deceptions, venturing to breath only the bubbles of truth as we find them.

And the plainest evil thoughts serve as inversions of the good — negatives images of true things and true colors. These don't deceive us much, and though they may contain scant (if any) fragments of truth, we can still use them for growth, because they repel us to the truth, to goodness, and to beauty. (Though, indeed, it is impossible to speak for long without saying something true.)

Thus, a God-hearted individual can use any thought to draw nearer to God, no matter what the intended meaning was. But, to say that God can make good out of any thought or meaning is not to say that all thoughts therefore are good in themselves.

It is not possible to speak of the inherent goodness of thoughts with the same quick affirmation with which we speak of the inherent goodness of things. We have a habit of conflating the thing with the thought in the case of creative works — and here, I think, is where your primary objection will be found.

The meanings which viewers make

But as I said before, it is impossible to speak long without saying something true (and, likewise, I expect, something false), and so in this sense, and with this optimism, I can, with a faithful heart, find God reflected in any creative work. But this idea of "reflection" depends more upon me, the viewer, than it does upon the work itself. That is to say, the good viewer can find good where it is scarce and make good where it is absent. I have seen the Spirit in me make good of films I would recommend burned and never seen by any human being evermore (and this is important to remember). But to say that therefore the work is good in itself because I could find good in it — I cannot affirm that, and I am not sure where to draw the distinction between the thing and the thought.

Perhaps, in the case of creative works, the things can be said to be good (the ink and the shapes on the paper, and the light and sound waves which compose the video and audio), in the same sense that all other things are good, but the thoughts can be said to be amoral, and thus their moral status depends upon our relationship to them (the author and the audience), so that, as Paul says, "if anyone regards something as unclean, then for that person it is unclean."