BuddyPress.org

You can use this setting in wp-config.php

define ( ‘BP_ROOT_BLOG’, 3 ); <– 3 should be your blog id that you want it to run on.

define ( ‘BP_ENABLE_MULTIBLOG’, false ); <– this will prevent the bp functions from trying to attach to the theme of a blog that doens’t have a bp enabled blog

Here’s the codex link:

https://codex.buddypress.org/how-to-guides/changing-internal-configuration-settings/