IN summary, your friend cannot practice with just her UK credentials in any state in the US. IN order to practice, she will need additional training at any of the American optometry schools from 2-3 years additionally. During my own training, I met a few UK trained optometrists. They all had to attend 2-3 years additionally.
Let's suppose that you wanted to practice like an ophthalmic technician for an ophthalmologist. This is possible. There would be no need for additional training or even a license,.