Your developer needs to use an acf/prepare_field filter using the field key varient. This filter is called before each instance of the field in the existing repeater.
$field['name'] will have values that look like this:
acf[field_61f2e6625ac01][row-0][field_61f2e66c5ac02]
acf[field_61f2e6625ac01][row-1][field_61f2e66c5ac02]
acf[field_61f2e6625ac01][row-2][field_61f2e66c5ac02]
etc...
This value needs to be parsed to determine what row of the repeater is being populated. There is also one that looks like this that needs to be ignored
acf[field_61f2e6625ac01][acfcloneindex][field_61f2e66c5ac02]
something like
// this code has not been tested
preg_match('/\]\[row-([0-9]+)/', $field['name'], $matches);
if (!empty($matches[1])) {
$row_index = intval($matches[1];
}
Using this information the value of the corresponding country field must be retrieved. I am not sure of the best way to do this. There is a chance that trying to read the repeater that is currently in the process of being displayed could cause the display loop to do odd things. I have never tried this. It may be necessary to use the field names in this instance an use get_post_meta() rather than using ACF functions. This can be done by constructing the correct meta_key
"{$repeater_name}_{$row_index}_{$sub_field_name}"
Then the choices of the current field can be populated based on the value of “country”
