It sounds like he's got the dreaded sprocket-flange wear ... the same problem I had.
If he has a standard sprocket flange, then it uses a circlip to hold a spacer and washers in place, to keep everything in line. The only problem is that the circlip groove wears and so you get free play in it. The next thing that happens is that your sprocket flange starts to grind down against the hub.
Changing to a new sprocket carrier won't fix this - you need a whole new sprocket flange using the new design that eliminates the circlip retainer with one big spacer.
The bad news is that these cost about 95 quid for the sprocket flange and another 7 quid for the spacer from Ducati. The good news is that you can sometimes get them from breakers for around 30-40 quid delivered ... I paid 35 quid inc p&p for mine.
The best way to ensure you get the right part is to ask for the sprocket flange and spacer from a 998 as these all had the revised spacer rather than the circlip.
http://ducatisportingclub.com/xmb/vi...d.php?tid=6972 [Edited on 13-3-2006 by antonye]