Answer: Unfortunately, summer is really the worst time to be planting because it is hot. This causes stress on the plants beacuse their root systems are not yet fully established.
Container grown plants can be planted in summer but you will have to be very careful about the watering. You do not want to overwater or underwater -- the soil should be kept moist like a wrung out sponge, not sopping wet or saturated and not bone dry. Try to plant just before a rainy spell if you can, and mulch your plants.
Establishing a lawn by seeding most likely will fail if you try it in the summer. The best time to seed a lawn is in late August to early September. (The next best time for seeding a lawn is early spring. These are the best times for planting in general, with the exception of evergreens which should be planted in the spring.)
But you can do all the soil preparation during the summer and have it ready to go. Depending on how well you prepare the soil and care for the lawn, and assuming you selected lawn grasses that are well suited to the local soil and climate, you should be able to have a decent lawn in a year or so.
I would suggest you work with your local county extension to run basic soil tests and determine how best to prepare the soil, develop a planting and maintenance plan, and also figure out which specific lawn grass varieties would do best in your parents' location.
Best of luck with your project!
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