I'll tell you the same thing I tell everyone with bad soil: The best way to have a good garden is to improve the soil with organic matter, compost, or well-rotted manure. The more organic matter you put into the soil, the more productivity you'll get out of it. Organic matter helps bind the sandy particles together so the soil will hold more water when it's dry, and drain more quickly when it's wet. If you can, add organic matter two or three times during the growing season. The more you add, the better. The sand might not be the only problem your friend has to overcome. The soil also could be especially alkaline or acidic. Do a soil test to determine the pH. The organic matter you add can help moderate extreme pH levels, but you may need other amendments that specifically deal with soil pH.