To make soup stock from standing rib roast bones, should the bones be roasted in the oven first or do you just boil them in water?

If there is any uncooked meat on the bone, toss in a hot oven to finish cooking then toss into your usual stock pot with usual stock ingredients (carrot, onion, celery, peppercorns, etc). I agree that they will give you a weak stock that you should consider pumping up with some low sodium beef broth, or even some demi glace. This stock would make a nice barley soup !

Rib roast bones will give up some flavor to a stock, but not nearly as much as, say, a chicken carcass. It might be best to think of the bones as a supplement to a stock that will get its flavor from other things---leeks, celery, onions, carrots and so on. It's definitely wise not to simply toss them out; I'm not sure I'd bother roasting them, though. A stock that's really beefy will need shin bones or other "soup" bones.

At this point roasting won't help much if they've already been cooked. The bones have already done their part for your pan juices. Also, don't boil soup stock. It should come to just below a boil and be held at a simmer for hours. Now you might want to roast your aromatic vegetables for the stock; carrots, onions etc. but that can be done separately.

They may not NEED to be roasted, but you will get a much better, browner, more flavorful result if you roast the bones at a high temperature until they carmelize, before making the stock. You can even add an onion or carrot or 2. Deglaze the roasting pan (even with just water) and add contents to your stockpot.