Sounds challenging. I like. 
In your includes/modules/<your_template>/pages/reviews/header_php.php file,
find:
PHP Code:
$reviews_query_raw = "SELECT r.reviews_id, left(rd.reviews_text, 100) AS reviews_text, r.reviews_rating, r.date_added, p.products_id, pd.products_name, p.products_image, r.customers_name
FROM " . TABLE_REVIEWS . " r, " . TABLE_REVIEWS_DESCRIPTION . " rd, " . TABLE_PRODUCTS . " p, " . TABLE_PRODUCTS_DESCRIPTION . " pd
WHERE p.products_status = '1'
AND p.products_id = r.products_id
AND r.reviews_id = rd.reviews_id
AND p.products_id = pd.products_id
AND pd.language_id = :languageID
AND rd.languages_id = :languageID" . $review_status . "
ORDER BY r.reviews_id DESC";
replace with:
PHP Code:
$reviews_query_raw = "SELECT r.reviews_id, left(rd.reviews_text, 100) AS reviews_text, r.reviews_rating, r.date_added, p.products_id, pd.products_name, p.products_image, c.customers_firstname
FROM (" . TABLE_REVIEWS . " r, " . TABLE_REVIEWS_DESCRIPTION . " rd, " . TABLE_PRODUCTS . " p, " . TABLE_PRODUCTS_DESCRIPTION . " pd, " . TABLE_CUSTOMERS . " c)
WHERE p.products_status = '1'
AND p.products_id = r.products_id
AND r.reviews_id = rd.reviews_id
AND p.products_id = pd.products_id
AND pd.language_id = :languageID
AND rd.languages_id = :languageID" . $review_status . "
ORDER BY r.reviews_id DESC";
Then, in your includes/templates/<your_template>/templates/tpl_reviews_default.php file,
find this peace:
PHP Code:
<?php echo sprintf(TEXT_REVIEW_BY, zen_output_string_protected($reviews->fields['customers_name'])); ?>
replace with:
PHP Code:
<?php echo sprintf(TEXT_REVIEW_BY, zen_output_string_protected($reviews->fields['customers_firstname'])); ?>
This should output the customer's firstname only rather than the full name.