What wonderful friends you have, and what a wonderful friend you are.
The passport card is really a waste of money for cruising. It offers absolutely no benefit over a birth certificate and ID, and will not help you get home if you miss the ship or have an emergency.
The standard warnings have been issued about traveling without a passport book. So if you have read them and are still comfortable cruising without it, just double check with your cruise line that your line and your particular itinerary do not require the passport. As long as that is the case, just have you