Hi Victor!  This is really hard but I'm really stubborn!  I can't use the frm_after_create_entry hook because I want this to happen on update.  So I thought I should try the frm_pre_update_entry hook.  I used that before to stop or allow an email to send successfully but not using a repeater. Below is the code I tested on an entry with just one repeater row.  I updated a name field in the repeater which this code is set to check.  It is basically the same as the first example here: https://formidableforms.com/knowledgebase/frm_pre_update_entry/

add_filter( 'frm_pre_update_entry', 'check_if_value_changed', 10, 2 );
function check_if_value_changed( $values, $entry_id ) {
	$display = "Entry ID " . $entry_id;
	console_log ($display);
	$field_id = 949; // name field in repeater
	$previous_value = FrmEntryMeta::get_entry_meta_by_field($entry_id, $field_id);
	$display = "Previous value " . $previous_value;
	console_log ($display);	
	if ( isset( $values['item_meta'][ $field_id ] ) && $values['item_meta'][ $field_id ] == $previous_value ) {
		// Stop email action if the repeater name field value did not change
		add_action( 'frm_trigger_email_action', 'frm_stop_email', 1 );
		$display = "No change in value, stop email from sending.";
		console_log ($display);
	} else {
		$display = "Updated value is " . $values['item_meta'][ $field_id ] . ",  don't stop email from sending";
		console_log ($display);
	}
	return $values;
}

What I discovered is that this code runs twice - first for the parent entry, and then for the repeater entry.  It does work, but I am testing a simple case first (only one repeater row).  My goal is for it to work if there are multiple rows and send emails only for each row that gets updated.  I'll try that next but I'm expecting it not to work that way because it appears that whatever the last result is on the comparison of previous and current values is the one that the action is set by.  I attached a screenshot of what was written to the console when I updated the name in the repeater entry.  Entry ID 363 is the parent entry (where the field values are empty because the field is not in that form), and Entry ID 362 is the repeater entry.

I gave up on the solution I was pursuing and came up with a different one that does not require PHP.   In a repeater in my form, there is a field for an email address, and a Yes/No radio field called Send email?.  In settings there is an email action that is set to "Run this action for" the repeater instead of the main form.  I want to be able to conditionally trigger the email action for select rows in the repeater on entry update.  Then by selecting "Yes" for the Send email? field on one or more rows, the email should be sent to only the email addresses in those rows. My solution:

1. Add a new field to the repeater to contain a copy of the email address, but make it conditionally display based on the value of the Send email? field.  Hide it from users by making it visible only to administrators.
2. Change the value of the TO: field in the email action to be populated by this new conditional field.
3. Add an API action to reset the Send email? field values back to "No" on update. I based this action on this example: https://formidableforms.com/knowledgebase/formidable-api/#kb-update-a-row-in-a-repeater

Now I can conditionally send emails to addresses in my repeater and automatically reset the trigger field in the repeater after each use.