Turbo Issues with Passat TDI 2.5 v6

[kwack]

A friend of mine in Madrid has the following problems which the dealer is quoting as a replacement turbio. As the dealer are basically useless, does this seem likely based on the symptons

"No whistling sound, nothing happens when your floor it.. works as normal upto 1750 rpm.. Its basically got no guts. But then no black smoke which is good..and it happened all of a sudden which make me think its not the turbo, as you would probably notice it getting worse and worse..

Any ideas what to check

Cheers

Kwack