I'd check the file on the server for possible corruption. I'd take a recent back up copy and use Filemaker 11 to run a recover on it and see if it has any problems. Since he's the "heaviest user" he may be encountering an issue with the database that others do not. The issue may only be apparent for FileMaker 11 users also...
Things to keep in mind about Recover:
- Recover does not detect all problems
- Recover doesn't always fix all problems correctly
- Best Practice is to never put a recovered copy back into regular use or development. Instead, replace the damaged file with an undamaged back up copy if this is at all possible. You may have to save a clone of the back up copy and import all data from your recovered copy to get a working copy with the most up to date information possible.