My neighbor just planted this hydrangea and is concerned about its appearance. She said one leaf looks like it has a silver dusting or color. Thoughts??

The image doesn't reveal much. It doesn't look like powdery mildew, but if it's a new planting it could well be the result of what we commonly call sunburn. Light levels well above what the plant is currently conditioned to tolerate can cause a chemical reaction in the leaves which results in creation of an oxidase you're likely familiar with - hydrogen peroxide. Like bleach, hydrogen peroxide is a very strong oxidizer and will attack (and "bleach) the first organic molecule it comes in contact with. Since the reaction that causes sunburn occurs in the layer of cells where the plant's photosynthesizing machinery is located, the green pigment, chlorophyll, becomes the direct target of the free oxygen molecules in hydrogen peroxide. The more scientific name for sunburn is photo-oxidation.
Snipping off the spent blooms will free up food/energy that can be utilized by the plant to increase the number of roots, then leaves and branches.
Al

It is too far away to tell from the picture. Grays, whites and silver growth usually suggest a case of powdery mildew. Or it may be some other causal fungal growth growing on top of leaf tissue causing the separation of leaf epidermis from the rest of the leaf.

Cut the string that attaches the leaf to the stem and recommend that she never water the leaves as dry surfaces are typically not favorable for spore germination. While not much can be done when the environment is typically very humid, ensure that the soil is not over-watered as that can raise humidity levels and promote spore germination. Instead, water only as soon as the soil is dry at a depth of 4" (the typical depth of new hydrangeas) and use sufficient water to moisten the soil at a depth of 8" some time after watering and waiting for the water to percolate. If she tests the soil at a depth of 8" and it is dry after watering, consider if the soil is not draining fast enough, if she missed a spot or if she used insufficient water (use more water the next time). If the soil is wet/soggy at a depth of 8" after watering, consider if she did not wait enough or the water to percolate or if she needs to use less water the next time.

The premature bloom browning as well as leaf edges that are browning inwards suggest that the plant was getting insufficient/inconsistent moisture at some point in the past. Aim to keep the soil moist always, especially now that summer is close. And maintain 3-4" of any organic mulch to conserve soil moisture, reduce weeds, protect the roots from hold/cold extremes and provide some nourishment as the mulch decays.