Welcome to the family, does this mean your not dealing with cancer, lucky you.  I'm going to make a couple observations about your question from a " mechanical" point of view, but I'd ask your vet to be sure. Obviously they shaved her, which means there is no protection on that part from the elements. Now, being from a farm and having to deal with cold weather and cows, I'm going to give you something I always had to consider in colder temps. When a cow is close to freshening(having a calf), there is swelling in the udder, and for first calf heifers, it is more of a factor, especially in the tits, which affects blood circulation, and when temps start to get into the teens and below, usually for extended periods of time, frozen tits becomes a concern and if they do freeze, it is usually on the ends. it won't show up for a few days, but if it happens can cause real problems, mainly because a scab forms then you have to pull the scab to get the milk out, but  what I am trying to say is that without the hair protecting her shaved area and probably being more tender from the surgery, frostbite would be more of a concern than infection. The next thing I would consider if your vet does say its ok to leave her out is clearing a spot so she doesn't have to hop through any snow, especially if the snow becomes crusted, and of course traction needs to be considered since those of us blessed to live in these parts knows what happens when snow gets packed, it's going to get icey, and you don't need any extra challenges right now, you are about to find your next couple weeks will give you all you want already. Good for you if cancer isn't a factor,once healed you will never know she's 3 legged except for the hop. Grab some sleep if you get the chance, it's something you might not get alot of these first couple weeks also. Don't be afraid to ask any questions,experience gives the best answers, Paws up, Spirit Gus and Dan