Try this:
In photo_popup_albums_config.php around line 40 change
Code:
WHERE userid = $userid
to
Code:
WHERE userid = $userid OR albumid = XY
where XY is the id of the album you want to share.
Also change around line 77
Code:
WHERE userid = $userid " . $where_add . "
to
Code:
WHERE userid = $userid " . $where_add . " OR " . TABLE_PREFIX ."albumpicture.albumid = XY
Untested, might work. Note that the images of the shared album will also show up if the user selects "Show all my images" in the albums selector.
Please let me know if that works.