He jsut has to add a dropper post...there was none at Sam's Club, we checked every aisle. But note, it does come with a kick stand where all the other suggestions here did not have that option, so he has that going for him.
I was going to recommend the kick stand but figured I'd better read the whole thread first to make sure I was not repeating someone else's suggestion & whoop, there it is!
Seriously though...
Full suspension, at least 130mm but if you plan to really enjoy the DH as much as the up I would go more than that.
Dropper post is a must.
Tubeless is great but not necessary starting out, just think about having "tubeless ready" rims.
Think about pedals, often not included in. I really like the security & efficiency of clipless (pedals you clip into) pedals. They scare some people but the learning curve is really quick. I am scared of flats.
27.5 or 29er? You gotta get on each & see which you are more comfortable on. There are pluses & minuses to each. I was never comfortable on the 29er when they came out & jumped on 27.5 as soon as they were available & loved it. I recently moved to a 29er as improvements in geometry have made it feel more to my liking. So far, it doesn't feel quite as agile in tight stuff but that is outweighed by the fact that I feel like I am on an e-bike a lot of the time. Kinda like the difference between a SL & GS ski.
Whatever bike you get make sure it fits YOU! A qualified mechanic/coach/sales person can help with this.
Factor in accessories. Hydration, spare tube, tools, bike specific clothing, helmet & shoes etc. Pump, chain lube, a brush & whatever else to keep your drive-train clean. Maybe a bike stand.
Begin with short, smooth, fun rides & build on success.
Enjoy the ride, that bike can get you to some pretty amazing places
\