A friend of mine recently bought a Freya, and he has a problem with it that we can't figure out.

The Freya shows no sign of any vibration during hover, and the tail is rock-steady, but if he adds collective a bit fast, the tail starts to shake sideways quite violently.

When the collective is reduced the shake goes away.

The Freya has a new GY401/S9253.
There is no binding between the S9253 and the tail (it's actually smoother than on my Sceadu, and mine works like a dream).
He has tried many different sensitivities on the GY401 with little or no result.

Anyone out there who might have a clue?

Make sure you have the acceleration and rudder mixes disabled! almost sounds like you have a mix in there that is causing problems.
Make sure that the boom supports are in the bottom holes .. I had problems with my Freya when the boom supports were in the holes that were recommended in the Manual.
Make sure that you are the proper distance out on your servo arm and not short or longer that what is recommended in the manual.

REVO-MIX is definatly off. He assigned it to a separate switch to confirm that.

Boom supports are on the bottom holes.

He used the same hole on the servo-arm as i did on my Sceadu. I don't remember the figures, but when i set mine up, the outer hole on a standard 4-armed servo-arm would agree with the manuals recomendations.

Check the belt tightness after the machine has been in the cold for a while. Perhaps it
becomes a bit loose.