I believe it's something like this:
foreach ($feed->get_items() as $item) {
$item->get_feed()->get_title()
// or if you need more than one thing from the feed like permalink
$parent_feed = $item->get_feed();
$parent_feed->get_title();
$parent_feed->get_permalink();
}
The documentation shows it being used in the feed constructor, but I use it this way.