1. Ask the user to carry out any changes in the site collection.
2. Try resolving his correct name by adding him to any any user group.
3. If the first way is not feasible because of your security policies and second is not working out to you then delete the user from site collection and add him once again.
What site collection means?
step2, any "AD" user group? or any SharePoint user group?
Step3, how to add/delete from what site collection/
Thank you.
Thank you