Index file growing endlessly - how to fix it?

I need to do some maintenance on a database on our client's site. Both log and index files were huge (Don't know how they took their backups to end this way) anyway I fixed the log file (.ldf) issue, log file is now merely a few megabytes (database is in simple recovery mode) The problem resides on the index file (.ndf) side. For comparison purposes, the database itself (data side, .mdf) is 265Mb. The index file (.ndf) is 57.3Gb!!

Every time I try to reorganise or rebuild indexes on this database, all available RAM is eaten up for this operation, the server becomes almost unresponsive and IO activity is incredibly high. On our test server, the operation wasn't ended after a full 8 hours activity. I can't do this on their server since this is a 911 emergency center.

I just wish to flush the whole index file data and then rebuild it properly. Is there any way to do this without having to render the server in an almost unusable state during the process and having to put the database offline?

Regards,

Nicholas