receipt_services_meta() is intended to display some HTML table of data. If you just want one value out of it, without a table, then you can't actually use the function.
But you can look at the function and mirror how parts of it work. Specifically, the bits that involve the $service_time.
$service_time is defined as
$service_time = get_post_meta( $order_id, '_wfs_service_time', true );
so the first question is: how do you get $order_id?
That came from
$order_id = version_compare( WC_VERSION, '3.0.0', '<' ) ? $order->id : $order->get_id();
so next is: how do you get $order? Because that was provided to the receipt_services_meta() function by something else.
If you're lucky, the $order from the first plugin may be the same as the $this->object from the second plugin. I'm skeptical, but without knowing more you might as well try it out.
$order_id = version_compare( WC_VERSION, '3.0.0', '<' ) ? $this->object->id : $this->object->get_id();
$service_time = get_post_meta( $order_id, '_wfs_service_time', true );