Lawprof - that 'expert witness' was very sure the helicopter was hovering or flying very slowly - not consistent with the debris field at the accident site.
I don't think you need a PHD to distinguish between an aircraft operating overhead or one transiting at 180kts.
Helicopters produce sound in a somewhat asymmetrical manner, without seeing it you'd have to have some talent to work out which way it was facing and how fast it was moving.
Even the best eyewitnesses are unreliable, let alone an earwitness.