The real question to pose to your debate opponent is:
By what authority does he claim to know enough about the nature of intelligence to say for sure what can or can not be accomplished without its help?
While science has neither explained the entire workings of the universe, nor disproved the existence of God, it has shown (so far) that God’s existence is not a necessary assumption to explain the universe as we currently understand it. That is, the universe seems to work quite well without assuming the existence of an outside intelligent force. That’s not to say that will always be the case, but so far, it has panned out as our knowledge has increased.
One religious leader (forgive me for not remembering his name) described this as the “God of the Gaps” fallacy. If you insist that the proof of God’s existence lies in the gaps in scientific knowledge, then as science closes those gaps, your case for God diminishes.
Science is an axiomatic system, and the mathematician Kurt Godel showed that any axiomatic system will not cover all truths. Suggest to your friend that he take comfort in the idea that God’s existence may lie in the realm of the unproveable but nonetheless true.