BuddyPress.org

Thank you!

Here is complete script for this:

// Setup
$path = 'some/folder/where/is/prepared/image.jpg'; // EXAMPLE
$user_id = 123; // EXAMPLE
$upload_path = bp_core_avatar_upload_path();
$upload_dir = 'avatars';

// Define filename and data
$filename = pathinfo($path, PATHINFO_BASENAME);
$data = file_get_contents( $path );

// Prepare final destination
$user_upload_dir = "${upload_path}/${upload_dir}/${user_id}";

if( !is_dir( $user_upload_dir ) ) {
	wp_mkdir_p( $user_upload_dir );
	
	if( !is_dir( $user_upload_dir ) ) {
		return false;
	}
}

// Prepare temp image
$img = "${user_upload_dir}/{$filename}";
$fc = file_put_contents( $img, $data );

// Get size
$image_size = getimagesize( $img );
$width = $image_size[0];
$height = $image_size[1];

// Save avatar and assign to user
$res = bp_attachments_create_item_type( 'avatar', [
	'item_id'   => $user_id,
	'object'    => 'user',
	'component' => '',
	'image'     => $img,
	'crop_w'    => $width,
	'crop_h'    => $height,
	'crop_x'    => 0,
	'crop_y'    => 0,
] );

// Delete temp file
wp_delete_file( $img );