Set Register Helper field to read only for members but not administrators or membership managers with the readonly field option.

my-pmprorh-read-only.php

<?php
/**
 * This recipe creates a custom fields for membership registration
 * that is readonly on the user edit profile for members and
 * editable by administrators or users with the membership manager role.
 *
 * This recipe assumes that the registration field is required during checkout.
 *
 * @requires Register Helper Add On
 * @link https://www.paidmembershipspro.com/add-ons/pmpro-register-helper-add-checkout-and-profile-fields/
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
// We have to put everything in a function called on init, so we are sure Register Helper is loaded.
function my_pmprorh_read_only_init() {
	// Don't break if Register Helper is not loaded.
	if ( ! function_exists( 'pmprorh_add_registration_field' ) ) {
		return false;
	}

	// Define the fields.
	$fields = array();

	$fields[] = new PMProRH_Field(
		'phone', // input name, will also be used as meta key
		'text', // type of field
		array(
			'label'          => 'Example Read Only', // custom field label
			'profile'        => 'read_only', // show in user profile
			'required'       => true, // make this field required
			'addmember'      => true, // shows on Admin Add Member page
			'memberslistcsv' => true, // include when using export members to csv
		)
	);

	// Add the fields into a new checkout_boxes are of the checkout page.
	foreach ( $fields as $field ) {
		/*
			Set to readonly for the user on the profile page if field is not empty.
			Administrators and Membership Managers are able to edit the field.
		*/
		if ( 'read_only' === $field->profile || 'read_only' === $field->attr['profile'] ) {
			if ( is_user_logged_in() && ! empty( get_user_meta( get_current_user_id(), $field->meta_key, true ) ) ) {
				$field->readonly         = true;
				$field->attr['readonly'] = true;
			}
		}

		// Add the fields
		pmprorh_add_registration_field(
			'checkout_boxes',               // location on checkout page
			$field                          // PMProRH_Field object
		);
	}
	unset( $field );

	// That's it. See the PMPro Register Helper readme for more information and examples.
}
add_action( 'init', 'my_pmprorh_read_only_init' );