If you inspect the return value of qa_when_to_html() (e.g. with print_r()) you'll see that the function indeed returns an associative array. The content should look somewhat like this if the date is less than $fulldatedays ago:
Array
(
[prefix] =>
[data] => 4 days
[suffix] => ago
)
or like this if it's more than $fulldatedays ago:
Array
(
[data] => Aug 20
)
So you need to display it somewhat like this:
$when = qa_when_to_html($time, @$options['fulldatedays']);
echo ($when['prefix'] . $when['data'] . $when['suffix']);