Thank you r-a-y for the illustration but I found another answer by Andrew the greatest
You need to do the following:
1) deactivate buddypress on the sub-blog
2) remove all buddypress tables
3) go into wp_sitemeta and remove all buddypress rows
4) remove any of the lines you’ve added to wp-config.php for buddypress
5) activate buddypress sitewide.