Source code for https://formidable-masterminds.com/repeater-fields-and-acf-integration

basic_function.php

<?php

add_action('frm_after_create_entry', 'populate_acf_repeater', 30, 2);
function populate_acf_repeater( $entry_id, $form_id ) {
	
	/* make sure we're working with the correct form *
	 * change 30 to the ID of your form              */
	if ( $form_id == 30 ) {
		
		/* get the entry header from wp_frm_items */
		$entry = FrmEntry::getOne( $entry_id );
		
		/* if this entry has no post, no reason to keep going */
		if ( ! $entry->post_id ) {
			return;
		}
	}
}

custom_code_complete.php

<?php

add_action('frm_after_create_entry', 'populate_acf_repeater', 30, 2);
function populate_acf_repeater( $entry_id, $form_id ) {

	/* make sure we're working with the correct form
	 * change 30 to the ID of your form
	 */
	if ( $form_id == 30 ) {

		/* get the entry header from wp_frm_items */
		$entry = FrmEntry::getOne( $entry_id );

		/* if this entry has no post, no reason to keep going */
		if ( ! $entry->post_id ) {
			return;
		}

		/* retrieve the repeater and row field ids by their keys
		 * this prevents issues when migrating code to
		 * new WordPress instance
		 */
		$repeater_id = FrmField::get_id_by_key( "demo_repeater" );
		$name_field_id = FrmField::get_id_by_key( "demo_name" );
		$city_field_id = FrmField::get_id_by_key( "demo_city" );
		$state_field_id = FrmField::get_id_by_key( "demo_state" );

		/* store the ACF repeater and fields names to variables for use in the loop */
		$acf_repeater = 'repeater';
		$acf_repeater_name = 'name';
		$acf_repeater_city = 'city';
		$acf_repeater_state = 'state';

		/* this returns the unserialized array of the repeater content
		 * that is stored as a serialized array in the wp_frm_item_metas table
		 */
		$repeater_meta_value = FrmEntryMeta::get_entry_meta_by_field( $entry_id, $repeater_id );

		/* get the number of rows for the ACF repeater field meta value */
		$number_rows = count( $repeater_meta_value );

		/* we can write the ACF repeater header record now */
		add_post_meta( $entry->post_id, $acf_repeater, $number_rows );

		/* let's loop through the array to get the data, format it, and write it to wp_postmeta
		 * the $loop_counter is necessary because it is part of the meta_key for each ACF row item
		 */
		$loop_counter = 0;
		foreach( $repeater_meta_value as $repeater_row_item ) {

			/* format the meta_keys for the ACF repeater */
			$name_meta_key = $acf_repeater . '_' . $loop_counter . '_' . $acf_repeater_name;
			$city_meta_key = $acf_repeater . '_' . $loop_counter . '_' . $acf_repeater_city;
			$state_meta_key = $acf_repeater . '_' . $loop_counter . '_' . $acf_repeater_state;

			/* retrieve the Formidable repeater field values
			 * $repeater_row_item is the entry id for each row entry
			 */
			$name_meta_value = FrmEntryMeta::get_entry_meta_by_field( $repeater_row_item, $name_field_id );
			$city_meta_value = FrmEntryMeta::get_entry_meta_by_field( $repeater_row_item, $city_field_id );
			$state_meta_value = FrmEntryMeta::get_entry_meta_by_field( $repeater_row_item, $state_field_id );

			/* write the ACF values to the wp_postmeta table */
			add_post_meta( $entry->post_id, $name_meta_key, $name_meta_value );
			add_post_meta( $entry->post_id, $city_meta_key, $city_meta_value );
			add_post_meta( $entry->post_id, $state_meta_key, $state_meta_value );

			/* increment the $loop_counter for the next row */
			$loop_counter++;
		}
	}
}