Your mechanic is wrong. There certainly is a prefilter on this engine.
See if you can locate the main filter. There is a large bolt that holds the main filter to the aluminum housing; the bolt head is visible on the top of the housing.
The pre-filter is located just to the inside (towards the valve cover) from this large bolt. The pre-filter is also inside the aluminum housing; it's held down by a C shaped piece of metal, which is held down by a bolt with a 10mm head.
If you replace the pre-filter, be SURE to replace the rubber o-ring. It usually doesn't come with the pre-filter, you have to buy it seperately.

Gilly