What is God to the non-dualist?
It depends on what you mean by “non-dualist”.
If you are referring to an individual, one who sees himself as a “non-dualist”, as different from one who is a “dualist” for instance, I have no clue about what God is to that one, we would have to ask her or him.
If your question to me is “What is God?”, I would say:
God is the reality of all things and all beings, it is the awareness which is truly perceiving and understanding these words right now, it is free from time and space, and it is time and space and everything in it.
We know it first as the Intelligence in our understandings, the Love in our relations, the Beauty in our contemplations, and the Happiness in our experience.
Then it reveals itself as the eternal reality of our human experience, as the awareness in which and to which it appears. At that moment, our mind-body becomes knowingly, willingly and happily the servant of this Presence.
Later we begin to recognize this Presence in and beyond everything we come to see, hear, taste, touch, smell, feel or think.
We were lost, and now we are found.
And we are the Finder and the Found.