Is it full sun? Is she at all interested in herbs? Does she want annuals that she'll replace each year, or perennials that can survive in pots outdoors over the winter (or be brought inside and stashed somewhere cool)?

The worst thing about potted plants in windy areas is that they dry out so fast, so you need to pick plants that can take that. Most silver-leaved plants are extremely drought tolerant, as are herbs. Lavender and rosemary are both good in pots.

One of my favorite flowering plants that grows in dry conditions is gazania; it has brilliant daisy-like flowers all season, and is available in specific colors and in mixes. It's not hardy, and has to be replaced each year.

I'm sure other people will have more specific suggestions.

I live in a very windy area with very sandy soil both of which require tough plants. I've had no problems growing marigolds, zinnias, and petunias. All three are annuals that are tough as nails and will flower all season. Several perennials would also fit the bill. Daylilies would thrive in that situation. I would recommend the ones that are everblooming such as Stella D'Oro, Apricot Sparkles, and Happy Returns (I have the first two). Coreopsis is another choice. I would consider an everblooming rose, too. Any member of the Knockout family (I have most of them) will bloom all season and not get too big in a pot.

Since your mom has a ready-made trellis of sorts on her balcony, you might also consider morning glories, or moonflowers or both. The former would give color in the morning and the other one would give white flowers and a heavenly scent in the evening. They could climb on the balcony and form somewhat of a windbreak for your other plants.

In general, plants with thick waxy leaves and those with fuzzy leaves are more tolerant of dry conditions as well as those with silver leaves as DtD suggested.

Along the lines of dealing with the drying out factor, you might also want to suggest that your mom add a product to the potting mix that is often referred to as water crystals. They are made of a polymer that when dry looks and feels like rock salt, but when water is added, absorbs much more (maybe 40 times) their size in water so that they look and feel like gelatin chunks. (It only takes a very small amount to do the trick, and before starting use of these, I killed many, many pots of plants.) The plants grow their roots to these little reservoirs which slowly release the absorbed water. It means that I only have to water once every day or two for my south and west exposure pots, rather than 2 times a day. Don't use little bitty pots, since they will dry out faster, and glazed or plastic pots will dry out more slowly than unglazed terra cotta or those coir lined wire baskets. Your mom's window boxes, unless lots deeper than standard, will probably only be useable for succulents since they just don't have enough soil to prevent drying out.

I also have grown thyme, lavendar, bay, and rosemary in pots and they have done well, though only the thyme and lavendar will survive the winter here, and the lavendar only if I tuck it out of the sun and wind so the snow will bury the pot (which I leave on its side for the winter so that it doesn't get too wet during a time when the plant isn't using water.) Your mom, being farther south and in an urban environment, may be able to keep all these except the bay plant alive outside.

I have grown pots of mixed succulents, both hardy ones, like some of the lower growing sedums or hens and chicks (Sempervivums) and annual or tender perennials like jade plants, echiveria, portulaca, many of which do just fine inside for the winter.

Licorice Plant (Helichrysum) is a silver (or chartreuse or variegated) stiffly trailing plant that does well in pots. Many of the flowering annuals like trailing petunias, million bells, lobelia, etc. won't do well in settings where they may dry out, so unless you get some other suggestions, she may have to stick mostly with plants with interesting foliage. Old-fashioned geraniums (Pelargoniums, not perennial geraniums) are amazingly drought tolerant, though ivy leaved geraniums aren't. I've grown both flowering geraniums and scented geraniums and wintered them over on sunny window sills and started new plants from cuttings. They may need more water (especially the scented geraniums) than some of the others, but are probably worth trying.

As for flowering perennials that she can leave outside all year, I'd suggest she try some southwest natives that like it dry and let the soil dry out between waterings. Some plants to try might include some of the hardier western Penstemons and Agastaches. The general rule of thumb for overwintering perennial plants in pots is that they should be about 2 zones hardier than plants in the ground for that zone, so in zone 7, look for zone 5 plants.

Other places you might have luck would be to ask in the container gardening forum on GW or to Google for images of "succulent pots, mixed." There are also books you might borrow from the library, like "Designing with Succulents" or various books on container gardening.

One final thought, though I haven't tried this: if your mom ends up with annuals in some pots, she can get bulbs in the fall and plant them after the annuals are done, leave the pots out for the winter, and start bringing them inside in late winter/early spring (maybe February?) to force and have some early spring color.

I have noted a house in an extremely windy location growing creeping jenny in window boxes off of their balcony. It grows into long (5'-8') stands of chartreuse leaves and is a very cool visual from a distance. It is an aggressive perennial thug here in zone 7 that easily winters over in pots. kt

Thank you so much to everyone for all the great suggestions! Don't hesitate to post if you think of anything else. Mom will be pleased.