It has been a long time since I have done that. So I don't recall what impact it had other than possibly he will need to wipe his cookies.
Anyway I would just look and see if there are indeed special characters.
That could be in member management or even become the member and see if you have any issues.
Of course if you do that you will need to ask him if it is okay.
Then I would just add a new account manually give him the login to see if he can in fact login for a test.
You never did say what was the issue with the login.
Like no error or invalid login error.
