pmpro-toggle-user-field-readonly.php

<?php
/**
 * Toggle user field readonly based on user meta value or user capabilities.
 * This should allow members to fill in a field at checkout but set to readonly
 * on their profile edit page if they are not an admin or do not have a value.
 *
 * This assumes a field's readonly property is set to true.
 *
 * 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/
 */
function my_pmpro_toggle_user_field_readonly() {
	global $pmpro_user_fields, $current_user;

	$readonly_fields = array( 'example_text' );

	// Don't break if PMPro is out of date or not loaded.
	if ( ! function_exists( 'pmpro_add_user_field' ) || empty( $pmpro_user_fields ) ) {
		return;
	}

	foreach ( $pmpro_user_fields as $field_group => $fields ) {
		foreach ( $fields as $field ) {
			if ( in_array( $field->name, $readonly_fields ) ) {
				// skip field if readonly is already false.
				if ( ! $field->readonly ) {
					continue;
				}

				// Remove readonly if user does not have a value for the field or is an admin.
				if ( ! is_user_logged_in() || empty( get_user_meta( $current_user->ID, $field->meta_key, true ) ) || current_user_can( 'manage_options' ) || current_user_can( 'pmpro_membership_manager' ) ) {
					$field->readonly = false;
				}
			}
		}
	}
}
add_action( 'init', 'my_pmpro_toggle_user_field_readonly' );

For setting default user fields, e.g. email address, to readonly see https://gist.github.com/ipokkel/9ea0a858661632d7f13c85b63299e846