How wonderful for your neighbor.* My son just returned 3 months ago so I understand how she feels.
Since she will want the plants to all be flowering at the same time, and since most perennials only flower for 2 to 3 weeks a season, I would suggest she plant annuals.* That way they will all be blooming all summer.* The biggest problem will be finding blue flowers as most are shades of purple.
Unfortunately you don't say what the sun conditions are.* Most annuals will do fine with part sun of 4 to 6 hours, but full sun will be 6 hours or more.* In your cooler zone you might be able to push the sun conditions with a thick layer of mulch and lots of compost added to the planting bed.* I would suggest you go to more then one nursery.* At a good garden center you could even ask the staff to contact you when any blue annuals arrive.** Sadly there isn't enough time to plant seeds and have flowers by the time he arrives.
This site sells seed and plants. Not all that I'm showing here are available from them, so call first.* You can search the main site by flower color, but there are pages and pages, many of which are perennials or short blooming annuals.* I've picked out those that have the longest bloom time or are annauls and should bloom all summer.* The ones with the two ** are more commonly found at nurseries.
**The first annual 'blue' flower that comes to mind would be Ageratum.* There is a tall and short variety.* They are perennials in warm zones, but would be annuals where you are.* The second one is a particular named variety, but will give you an idea of the habit of the shorter plants.* They prefer part sun.
Brachycome comes in different colors and you might be able to find blue.
Gilia might also work for you.
Laurentia might not be in bloom in time, so ask if you intend to order.
Blue flax aka Linum might work for you.* Linum grandiflorum
'Blue Dress' is a taller version.
**Annual blue flax aka Linum usitatissimum “Annual Blue Flax” would work, but they don't have it in production.* Here's what it looks like.
**Lobelia is commonly offered by most nurseries.* It comes in a range of sizes and colors of white to pink to purple to blue.* You'll want the annual bedding plants and you can usually purchase it in flats.* Here's a blue version.
Lobelia erinus ‘Fountain Blue’
**Nemesia is also commonly found in nurseries.* There are shades of blue.
Nemesia strumosa ‘Blue Jay’
Phacelia might work, but can cause skin irritation and might not be a good candidate for kids to plant or weed around.
Wahlenbergia could work as well.
Keep in mind that I've selected those that would be annuals in your zone so you'll have a long season of bloom.* Here's their main page where you can search by color if you like.
I haven't included red or white flowers as annuals of these are easily found at any garden center.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.