My well has salty water, what can I do?

Don't try homemade desalination units because they're not going to work - reverse osmosis needs expensive equipment and constant maintenance that's not worth it for a well. The most practical thing is to drill at another spot on your land, preferably at greater depth or in an area farther away where the groundwater is less salty, but first call a hydrogeologist or someone who knows about local conditions to tell you where you're most likely to find fresh water. If the salinity is moderate, you could use it for irrigating salt-tolerant plants or for other non-potable uses while you figure out the new well situation.

Have you already checked whether the problem is just in the area where they drilled or if the whole property has that salinity level? Honestly, when I had that same issue a while back, a local geologist told me that a lot of times salt is in specific layers of the ground and by drilling deeper or a few meters to the side you can find fresh water. Before you spend money on desalination or a new well, it'd be worth having someone check the geological profile of your area to understand what's underground. If the salinity is widespread across the whole property, then yeah, you'll probably have to drill somewhere else or think about bringing water from another source, because home treatments aren't going to solve a problem that big.