You can't do this ... well, not easily, anyway. This question is also tied up with a
you asked less than four hours ago, isn't it?
I have said this, and others have also said this, on many occasions: Kunena is not responsible for how users register or login to your website. That's the start, end and the middle of the discussion. Joomla handles the way your users register and login. The Joomla registration process may be helped by third-party extensions like Community Builder or JomSocial.
Having said those things, what does your request for a separate "login module to my forum that is not connected with the Joomla login (nor connected to the Joomla user database)" mean? You see, that question is impossible to answer in a Kunena context.
G'day, jcolome. As I have said on many occasions, Kunena is not responsible for user registrations. This is a Joomla matter. Your question has probably been asked and answered dozens of times at forums.joomla.org.
Solution: Change your user record (at the site you constructed on behalf of your friend) site so that you don't receive "system e-mails".