Will make all the files in the path owned by you. Be careful if you have more complex permissions on there they will be lost.
This is where ACL permissions would help. He could give his new id ACL permissions to the files and that wouldn’t mess with the current permissions.
From the root/beginning subdir: sudo setfacl -R -m u:{replace with your new id}:rwx .
Honestly that’s a lot of windows users. I have to deal with it at work because we are ‘a microsoft shop’. Ughh. I am in a relationship that I cannot get out of, sort of an arranged marriage. Retirement can’t come soon enough.