I would say that a person from Phoenix would not need to be terribly prepared for the weather in Paris if they were going in the summer. In that season, Paris would be a wonderful change for them. In any of the other seasons, I would need to warn them to prepare for colder and rainier weather than they are used to.
In the summer, the weather in Phoenix is not very pleasant. The average high in August, for example, is over 100 degrees Fahrenheit and the average low is only in the low 80s. This is a very hot place. The average high temperature in Paris in August, by contrast, is 75 degrees Fahrenheit while the average low is 59 degrees. A person from Phoenix might need to be reminded to bring a light jacket, but in general, they would surely like Paris better than home.
If the person were going in the winter, by contrast, they might need to be warned to buy warmer clothing than they already own. Even in Phoenix, the average high in January is 66 Fahrenheit. The average low is only 44 and the average low is 35. In Paris, the average high is only about 43 Fahrenheit. The person from Phoenix might not have clothes meant for that kind of weather.
Paris, France, also has much, much more rain than the arid state of Arizona; therefore, the purchase of a raincoat, or, at least, an umbrella may be in order. Perhaps, too, the tourist may wish to bring some shoes that can be worn during wet weather. While France does not have the extremes of temperatures and weather as do many of the Northern states of the U.S., there are fluctuations in temperatures that would certainly be different for the tourist from Arizona. So, bringing a lightweight sweater is in order.
Sometimes, too, there is wind; and, depending upon the season, this wind can add quite a chill to the tourist. So, perhaps, the tourist may wish to bring clothes that can be layered, such as varied shirts and sweaters. For, the air of Paris is certainly damper than the dry air of Arizona. With layers, the tourist can "peel off" some of his/her excess clothing.
Finally, tell this person to have a good time. Paris, "the city of lights," is a beautiful city, and the evenings are quite romantic and lovely,especially along the Seine river. Of course, as is said, On mange bien en France--one eats well in France. So, suggest that the tourist bring loose clothing (chuckle).