Hi Michele,
As per your description the issue is only with his user. Then, please go to transaction RSRT -> put the problematic query there -> click on "Java Web" button. When the browser opens, add &USE_PERSONALIZATION=false parameter to the URL.
It will run the query for the user without personalization. I assume this is the issue. If it works when using this parameter, then go to table RSWR_PERS and delete the entries for the specific user/query.
Cheers,
Marcio