Index 255 is the Large Object data for a table. For sysindexes, it's going to be the data in the column statsblob, as that's the only LOB column in the table. Fortunately, that's not critical, it's the statistics info for the indexes in question. Hence you're unlikely, from what I can see, to lose any data because of this (unless there are other corruptions). However I don't believe this is repairable. CheckDB will not fix the system tables and trying to do anything manually is likely to cause worse problems, seeing as it appears that some of the indexes on the system tables are damaged.

Suggestion:

Create new database, script all tables and objects and create them in the new DB. bcp all data out and reload into the new database.

Perhaps point out to the client that ignoring corruptions is not a clever thing to do, regardless of whether or not it seems to be causing problems. Had this been picked up shortly after it happened, a backup could have been restored and the log backups rolled forwards.