I second what Contender said.
I'm not a brand snob and can honestly say that most of the "name brand" AR platform rifles/carbines will perform adequately for most users. The AR is unique in the firearms world as it is so adaptable-often called the "gun guy's LEGO set".
With that said, it is quite like picking out shoes and just grabbing one off the shelf is likely to create some level of "mis-fit". A gift certificate might be the best way to gift something like although it does take away from the WOW factor of opening a long narrow box.
I feel comfortable recommending these brands: Palmetto State Armory, DelTon, CMMG, Ruger, and S&W. The AR supply chain is full and it's a buyers' market so some careful shopping should put a new owner in a decent quality AR with some accessories for that $600 figure.
Prior to the last election, I sold a slightly used AR to a family friend to fund another project I had in mind. The AR, 5 magazines, 1000 rounds of ammo, a used $200 red dot optic, a "tactical" case, zeroed and ready to use cost him less than $800.