It doesn't seem very sensible to call a claim that someone "ought" to do something "false" if you're denying that an "ought" claim could ever be meaningful in the first place.

Anyway, it's a very annoying argument. It seems an awful lot like saying "You can't prove there's a such thing as value, therefore I refuse to take your money."

I'd be tempted to respond by hitting him with a stick until he conceded that stopping getting hit by a stick was a sufficient motivation to do X.