Change the 'Attendee 1' text to 'Purchaser/Attendee 1'. This will only change the text on the checkout page and only applies to the first registration (where 'Attendee 1' is used for the legend)

example.php

<?php //Please do not include the opening PHP tag if you already have one 

function tw_ee_form_section_proper( $options_array, $form_object ){

    if( isset($options_array['html_class']) && $options_array['html_class'] === 'ee-reg-form-attendee-dv' ) {
        if( isset($options_array['layout_strategy']) && $options_array['layout_strategy'] instanceof EE_Fieldset_Section_Layout ) {
            $layout_strategy = $options_array['layout_strategy'];
            if( $layout_strategy->legend_text() === 'Attendee 1') {
                $layout_strategy->set_legend_text( 'Purchaser/Attendee 1' );
            }
        }
    }
    return $options_array;
}
add_filter('FHEE__EE_Form_Section_Proper___construct__options_array', 'tw_ee_form_section_proper', 10, 2);