Hi Luca,
It will relate to the configuration. I think you can try this:
1/That user must belong to a User Group with unique access level, you can assign JA Megafilter menu item to that access level specific.
2/ You can set that user's K2 items to a K2 category and in the filter settings of JA Megafilter, you can set to show K2 items from that K2 category only.
Regards