There might be 2 things happening here.
a. He is using a file format that doesn't support performer tags and it is being displayed in the tag diff in the ui (Recently fixed - see PICARD-253)
b. Some issues with id3 compatibility and performer tags.
Both were recently fixed and will be released soon with 1.4
If it is a different issue, please make a ticket on JIRA with the picard version/ file info and MBIDs and I'll look into it