"He has 3-4 client meetings per month."

Does he have a meter on the flush mechanism, and is keeping a recorded log?

That's a pretty expensive remodel for that little use.

What about this part: "made by the taxpayer to an interior portion of a nonresidential building placed in service after the date the building was placed in service"

And unless he's going to take it with him when he moves, why would you want to start depreciation on this?

"how can business-only use be defended?"

Is it specialty? For example, ADA compliant? His clients have special needs, and this remodel provides what was lacking? Is there an employed attendant?