Helical waves in the tail of Comet Morehouse are studied in the WKB approximation with the plasma non-uniformity along the tail taken into account. A comet tail boundary is considered as a transition layer of thickness d, with the linear velocity profile within it. This approach enables us to set the limits of applicability of the tangential discontinuity model: for wavelengths λ which satisfy the condition 2πd λ < 10-2 both models are practically coincident. The thickness d of the tail boundary is derived from the observations of the increase of the wave amplitude along the tail axis: d ≈ (15-20) × 103 km. With such a thickness stability conditions turn out to be marginal. This could explain why helical waves in type-1 comet tails are seen only occasionally and not at all times.