In general it won’t matter too much as all cards are based on the same reference design.
The non-Nvidia aftermarket cards generally have better cooling though.
In terms of clock speeds there is again not too much in it. If he knows how to overclock etc he should be able to boost pretty much any 1080 ti to be on par with the others out there.
There are however versions which come overclocked straight out of the box which saves on the hassle of doing this manually.
Some cards are superclocked, and are often selected (or binned) to be the best performing of all of the cards in a given batch.
IMO the ‘best’ you will find is probably the Zotac 1080 ti AMP Extreme edition, as it has one of the highest clock speeds straight from the box. Zotac are a great brand for reliability too.
That said I would think it will be difficult to find any 1080 ti in stock atm, especially for a reasonable price. As such, any you find will perform well.
Also, bear in mind that some of the cards are massive (length/width wise) and will struggle to fit into smaller computers and systems. They usually have the measurements on the manufacturers website, so just make sure you measure up before buying.