//Create ACF field called `widget_class` and assign to all widgets to use this function

add_filter('dynamic_sidebar_params', 'my_dynamic_sidebar_params');

function my_dynamic_sidebar_params($params)
{
    if ($params[0]['id'] != 'footer') return $params;

    $widget_id = $params[0]['widget_id'];

    $classes = get_field('widget_class', 'widget_' . $widget_id);

    if ($classes) {

        $params[0]['before_widget'] = str_replace('col-xs-12', $classes, $params[0]['before_widget']);

    }
    return $params;
}