When you look at what he spent on the ES vs what he'd be able to get for it now he'd take a huge financial bath. Unless the LS were a spectacular below market deal he'd definately be in the red.
I'm sorry, but even if it is a Lexus its still a car. If you go through life just changing your mind months after buying a new car and flitting around to new options all the time, unless you have significant financial resources you will damage yourself.
As for the car, the ES is far more LS like than most people (namely LS owners) like to admit, ESPECIALLY the ES350. The ES post 02 was designed specifically to posess much of the driving feel and dynamics of the LS. The LS is a nicer car, no question and given the choice between an LS and an ES I'd take the LS (although I would still probably, and did, choose a new ES over a used LS). But unless you're switching back and forth between the ES and the LS all the time you don't notice the areas where the ES falls short of the LS. The LS is the car that got me into Lexus, my dad has had two. When I drive the LS I'm always awe struck by it and immediately start thinking about buying one for myself. Then, when I drive the ES again I'm always impressed by how similar it is to the LS and the desire to replace it goes by the wayside. Whenever my Dad rides in or drives my ES or an ES350 loaner he always says the same thing, that he's surprised how similar it is.
The switch isn't worth taking a bath for.