It may be that all future conversations you have with your friend might no longer lean towards such weighty philosophical considerations, but I just briefly wanted to deal with his question as to why God couldn’t just be reasonably great, instead of having to be maximally so.
I think at least one of the reasons why God could not be considered as a being who is only “reasonably great” would be because, to begin with, mere “reasonable” greatness would not be a greatness sufficient enough to create beings with the capacity to conceive of something beyond it. We can, for the most part I think, safely say that men and women can and do carry the attribute of reasonable greatness, as many truly great things have been achieved as a result of human effort and ingenuity. But men and women, along with all of the other existing entities within the universe, still permanently possess the attribute of contingency and must themselves, therefor, irrespective of such greatness, however instantiated, be dependent upon that which is not contingent, which logically takes to us the word necessary. God can logically be shown to exist as necessarily, that is to say that he exists in the ‘necessity of His own nature’, as formulated in ‘the argument from contingency’
This short video link below gives a summary of the argument
Once we can see why a being who exists in the necessity of his own nature is the only thing logically adequate for the creation of contingent entities existing in the universe, a question we can can ask ourselves in regards to why God has to be thought of as a being who must have maximal greatness would be:
Can a being who exist necessarily exist as anything less than maximal?
A state of existence which is, in the absolute sense, independent or non-dependent, would have to be thought of as a maximal state. Contingency is not a maximal state, it is a dependent state. Contingent entities, such as humans, exist with an inherent dependency on a myriad of factors that work together to ensure the continuation of there existence, things such as food, the sun, water, and the very air that we breathe. Because of this you can not say that humans exists maximally. Rather our potential in every area is limited and constrained by the physical world. This extends into everything in the contingent world. Nothing within it exists maximally
Things within the contingent world can not exist maximally because they do not exist necessarily
God however does exists maximally precisely because he exists necessarily, which would also logically entail that all of Gods qualitative attributes, such as greatness, would have to exist maximally as well.
The philosophical fact that a necessarily existing being would have to exist maximally helps to secure the argument against the charge of the word “maximal” being a word smuggled in arbitrarily.
I hope that this gives you a different take on some of the things your discussing, which are very difficult and require great amounts of thought and patience. Please Keep us posted as you are able as your conversation continues, and my prayer is that ultimately this individual recognizes his deep and abiding need for the love of Christ which never fails, even when arguments can and do-
Love never fails. But where there are prophecies, they will cease; where there are tongues, they will be stilled; where there is knowledge, it will pass away. 1 Corinthians 13:8