I would give him his own file. He can create layouts as he wants without damaging yours and you can control his privileges. I would also get some protections in writing and add more frequent backups.
You are right to worry ... it is one thing to allow him to create their own reports in separate file but he should NOT be changing data without proper interface in place. Have him purchase FM DataGuard which will allow incremental roll-backs ... and I truly wish you the best.
Oh I meant to say also that script cannot fire in layout mode. You can use FM-diff to find differences between files but if someone moves objects on a layout I am not sure even FM-diff could tell you which user did it.
+1. With the ability to add tables to "the graph", you could give them a "report file" with full access to the necessary tables. They can't define or modify fields, but they can create layouts based on the fields and even create scripts, relationships and value-lists!
Thanks everybody for your answers. If anybody knows a logging plugin that would explicitely log changes in Layouts, I think this might be another option for me.