To show all commits for a certain user on the current branch: git log --author=JohnDoe
To show all commits for a certain folder: git log -- path/to/folder
No combine these two commands: git log --author=JohnDoe -- path/to/folder
To keep it more readable, you can also add --oneline, to only show the commit hash and title.
PS.: It´s important to leave a blank between -- and /path/to/folder!