How should I caulk clapboard?
I don't think your friend's suggestion is bad, but I'd still want to be sure the caulk extended all the way to the exposed surface of the siding.  Better not to let water get into the crack at all than to let it get in and stuck between siding and trim but blocked from penetrating further by caulk.

Why?

Because in winter, water that's in the crack will freeze, expand, and make bigger cracks.  Water on the surface can do no harm.