Hi, I'm seasoning and cooking a prime rib for 10-12 servings. One person can't have garlic. Can I cut one piece off and cook it separately in f c.

Another approach.
Roast the whole piece of meat without garlic.
Make as a side dish roasted garlic and serve it at table as a garnish...whole or pureed as a sauce with optional more olive oil and/or good red wine vinegar.
Voila - no problems with timing roast or garlic aversion.

Is it all aliums or just garlic? If it’s garlic alone, you could lay split leaks in the roasting pan.

Can you use other seasonings besides garlic? I’m the person who’s sensitive to garlic (I wish I weren’t because I love it). I use salt, caramelized onions, Worcestershire sauce, black pepper, sometimes lemon or vinegar, and sometimes horseradish, depending on what I’m making, to make food taste good without garlic. Can the person have the essence of garlic? Like you could roast the prime rib with whole pieces of garlic in the pan that you don’t serve to the sensitive person? I’ve also found that I can have garlic if I take the little bud out of the center of it and then cook it really well. But your friend's needs might differ. I hope this helps you.

You can, the cook time will be shorter which also means it won’t brown as much. You may want to sear it first then finish it in the oven. Are you following a recipe? Perhaps if we see it we can offer more advice?