function format_capabilities( $value, $field_type, $field ) {

    if ( $field == 'my_field_key' && is_array( $value ) ) {
        $value = implode( ", ", array_keys( $value ) );
    }

    return $value;

}

add_filter( 'wpf_format_field_value', 'format_capabilities', 5, 3 );