Including the extractor itself, there are a total of 6 parts ...
Extractor
Striker Safety Plunger & Spring
Extractor Plunger & Plunger Spring
Extractor Spring
There are a couple of little tricks to installing the parts back in the slide in the easiest order, so it probably ought to be done by a local gunsmith who is familiar with the P99 series and doesn't mind taking a minute or two and doing it for your friend.
The safety plunger can be installed incorrectly, turned 180 degrees the wrong way. (The "tooth" should be toward the outside edge of the slide.)
Also, S&W changed the orientation of how the extractor spring is to be installed in the SW99's some time ago. Instead of the spring being installed with the large end fit into the slide's spring hole (tight fit), the spring's orientation was changed so the small end goes into the hole (loose fit). This requires some attention to make sure the spring remains in its proper position, and doesn't slip, while the extractor is being installed in its recess over the extractor plunger and the safety plunger.
How experienced is your friend with disassembly & reassembly of this gun?
The parts can be ordered from S&W (Walther America).