Redirect user to their primary site dashboard after login on the main site

Hi !

On my multisite installation, people can create a subsite (with Pro Sites activated). For example, USER1 will login on the main site login form. After that, he sees a page that tells him that he can't access the main site dashboard but can access the dashboard of his own sites.

I don't want that... I want them to be automatically redirects to the dashboard of their primary site (only if they log from the main site).

Could you please help me ?
Thanks !