BuddyPress.org

Are you sure you have your hook setup correctly, and that you are definitely calling the delete account code?

the bp_core_delete_account() function calls on to wp_delete_user() and should in all certains call the delete_user action.

Is the user getting deleted, i.e. can you confirm the delete routines are getting called correctly. Are you deleting from wp-admin and confirming the deletion?