You want to use a rice with the right amount of starch and which will let it go in the proper way at the proper time. Arborio is the classic, but several of the short and mid grains work just as well. CalRose is my favorite.
You want to avoid long grains such as generic long grains, Jasmine and Basmati which don't give up enough starch, and also glutinous "sweet" rices, which give up too much.
You and your partner used at least one of the wrong rices. Let me get this straight. He added new, shorter grain rice partly through the cook? Oh Jeeze!
The right amount of stock is however much it takes to reach the right consistency and doneness. Since you add in stages, that shouldn't be a problem.
Your rice was too hard because it was undercooked. Ordinarily this results from pulling the dish off the stove too soon because the cook is anxious. However, if new rice was added partway through your cook, that's the explanation. In any case, trust your mouth, not your fears. Next time, add a little more stock and keep cooking and stirring until all the rice is done. Underdone rice is a bigger sin than broken down rice. Risotto should be like a creamy porridge to some extent, otherwise it would just be wet rice.
The usual problem with risotto is overcooking. People tend to cook it until it's done, then a little extra to make sure, then they take it off the stove and the risotto "coasts" further still; it tends to be over thickened and sticky. Ideally, you want to take it off the stove when it's still a bit soupy and just an instant before it's done. It will take more than a few risottos to get the timing down.
Get a new partner.