The concept itself must be a concrete thing, otherwise we could not conceive of it.  We are things, and since we are able to perceive the sensations the concept is modeled after and there is a causal relation between the two, then the concept must exist in the physical world in order to be derived from it. 

That is, of course, unless we decide to be dualists. 

Love may be an abstract term, but it is a concept based on something we actually feel, not see in the gaps of our feelings.

We do have concepts for things in the gaps of our experience.

That's all God is; a concept for the lack of a concept.  There is a lack of a concept because there is a lack of sensation; a lack of evidence. 

Does that mean it doesn't exist? Not necessarily.  But until I have the evidence or reason to believe there is even the possibility of evidence (how could one have evidence for the supernatural if the supernatural is, by definition, not-natural, and evidence is natural) why should I believe?

Tell him that.