BuddyPress.org

See template entry.php

https://buddypress.trac.wordpress.org/browser/tags/2.3.3/src/bp-templates/bp-legacy/buddypress/activity/entry.php

Caution: modification are preferably done via child-theme.

If you use Dan’s approach, you want to do it by first making a template overload of the file he links to. And then writing a conditional for the calls to the user link. It will work fine.

Or you can use the filter hook for bp_activity_user_link() and then you don’t need to touch template files. You could write a filter in theme/functions.php – something like:

function imborx_profile_link( $link ) {

   if( $link != bp_loggedin_user_domain() )
       $link = // custom url

   return $link; 
}
add_filter( 'bp_get_activity_user_link', 'imborx_profile_link', 15, 1 );