Babs... I'm going to disagree with this, UNLESS she has excellent quality soil to start with. You want the tree roots to REACH OUT for nutrients... if you plant them in very highly amended soil (good compost), they tend to "look inward"... actually stay within the hole, and when they exhaust the easily available nutrients, they're already on the way to becoming root bound.
When digging a hole to plant a tree, you want it to be larger than the root system (at least double diameter). You also don't want a "round" hole... preferably, you want multiple jagged spots reaching outward to help guide the roots out, away from the trunk. If you have clay soil, this is especially important, as smooth walls to the hole actually creates a "flowerpot" effect.
Replace the soil with what you removed. THEN, mulch with some well rotted compost, from about 8" out from the trunk to the "drip line" (the tips of the branches). This will slowly percolate nutrients into the soil near the roots, which will "grow out" looking for more.
And yes, given your temperatures and season, you'll really need to stay on top of watering... you need to water DEEPLY every few days, not sprinkle the surface daily! And again, mulch will really help hold the moisture.
Summerthyme