The struggle with evil has probably hit every Christian pretty hard at some point. I guess this is why Christ gave us the Beatitudes.
However, you might want to point him in the direction of St. Augustine’s Confessions. He struggled with evil as well, and although he was a Manichee (gnostic sect that posited God as a material being who had an opposite evil that positively existed, and matter in itself was evil.) Augustine adopted the Neo-platonic doctrine of being, which essentially was that the whole of existence is a gradation of being. The closer something approaches to God, who is Existence, the more perfect it is. The further away it is from God, the less perfect. He points out, as he said he was plagued by questions of the existence of gross creatures like worms and the like, why would God create such things? He determines that it was because the perfection of the whole requires lesser beings – as modern biology is showing. If you don’t have amoeba’s (sp) in the ocean, there is no life in the ocean. They are all interdependent.
This gradation of being also allows Augustine to determine that evil is none other than a privation of the good. Evil has no positive existence, though Aquinas points out, to our minds it has positive existence (e.g. evil is a being of reason, an 'ens rationis.) This is all very abstract, but it helped me when I was struggling over the same question.
As an example, you might point out some crime, like murder. Murder is certainly an evil, but what gives it its existence? The fact that it is the taking away of the good that is life. Thus it is a privation. Why does God allow it? I always say he is committed to our freedom. It would be easy for Him to reveal Himself to us in all His blessedness, as He is the Sovereign Good, and we would immediately be drawn to HIm. Anyone who sees goodness in itself can’t help but choose it. However, then God would have a bunch of automotons on His hands, and no one would have chosen Him for His sake, that is, nobody would love Him, at least for the right reasons.
Augustine discusses all of this in the 7th book of the Confessions (not the murder example that was off the top of my head), but in the same place he describes how matter in itself is good. He says since corruption can take place in matter that proves that there is some good in it – if it were no good at all it could not be corrupted. If it were a supreme good (e.g. immaterial like God and the angels) it could not be corrupted, so it certainly is not a supreme good. Therefore, matter is a good but not a supreme good.
This little argument helped me because it showed that matter is good, but inherently corruptible. Therefore, it seems to me that God could not create a material order without corruption, that is, corruption goes with the territory, except in the case of man. In the case of man, the form of his body (i.e. the rational soul) is immaterial, and before the fall of Adam and Eve, it somehow prevented the material side of man from being corrupted, as the soul is intricately bound to the body. Aquinas even goes so far as to say that Eden had the prerequisite material conditions that kept the elements of man in perfect balance, thus not allowing him to be corrupted. Aquinas says when man was expelled from Eden, the material elements were then not in proper balance. This is all conjecture, but at the Fall, however, the body “rebelled” and took partial control of the reason, and hence, death entered as punishment.
So, the short answer to your question is to get him to understand the neo-Platonic understanding of gradation of being. Ettienne Gilson in his Spirit of Medieval Philosophy, has a great example. He says to imagine creation as an intricate symphony, where the rising and falling of animals are like the sounds in that symphony. THe only way that symphony reaches its perfection is by the notes rising and falling as they do. The same goes for creation.
As to God “needing” worshippers, this is absolutely absurd. Aquinas points out in the Summa Contra Gentiles that God’s will is eternally of Himself. This is the object of His will, now the means to attaining that object are manifold. He could be complacent with willing Himself eternally and this would be a sufficient condition for him. But as the medieval philosophers pointed out, ‘bonum diffusivum sui’ (the good is diffusive of itself). Aquinas said that God’s essence was His existence and that he was “esse subsistens.” Creatures’ essences are distinct from their existence (what they are is distinct from that they are). This distinction between God and creatures is helpful in understanding, that God doesn’t need creatures. For he is full of His own perfection having existence in its fullest, that he does not “need” creatures as they add nothing to Him. He merely made creatures because He is good, and the good diffuses itself (ever wonder why the sex drive is so powerful, well its because we seek to share the good that we are by creating offspring.) God demands that we worship Him, not because He needs it, but because He deserves it for what He is and what He’s done for us. Just as a father deserves the respect of his children for pro-creating and caring for them.
I hope this helps, and sorry it is so long winded. It’s tough to answer such a deep question with a short response.