Fatal error

This one is definitely a bug. I think wpuf_user_edit_profile_form() should be $edit_profile->show_form() in wpuf-editprofile.php.

To change yourself find the file wpuf-edit-user.php and change the function wpuf_edit_users() to this:

function wpuf_edit_users() {
    global $edit_profile;

    ob_start();
    //if user is logged in
    if ( is_user_logged_in() ) {

        //this user can edit the users
        if ( current_user_can( 'edit_users' ) ) {

            $action = isset( $_GET['action'] ) ? $_GET['action'] : 'show';
            $user_id = isset( $_GET['user_id'] ) ? intval( $_GET['user_id'] ) : 0;
            $userdata = get_userdata( $user_id );

            switch ($action) {
                case 'edit':
                    //if user exists
                    if ( $user_id && $userdata ) {
                        $edit_profile->show_form( $user_id );
                    } else {
                        printf( __( "User doesn't exists", 'wpuf' ) );
                    }
                    break;

                case 'wpuf_add_user':
                    wpuf_add_user();
                    break;

                default: wpuf_show_users();
            }
        } else { // user don't have any permission
            printf( __( "You don't have permission for this purpose", 'wpuf' ) );
        }
    } else { //user is not logged in
        printf( __( "This page is restricted. Please %s to view this page.", 'wpuf' ), wp_loginout( '', false ) );
    }

    return ob_get_clean();
}