Never ridden within 1000 miles of Michaux, but i'm guessing rocky, rooty, twisty? I'd go short travel not too slack 29er. If your son's girlfriend is 5'4" or taller, I'd go 29er for her too. Not sure how much you want to spend, but if you are in the XC/trail FS category I think an alloy bike is just fine, and contrary to crownjewl, I'd rather have an alloy frame with a good fork and shock than crappy suspension on a carbon frame. MTB carbon is pretty overbuilt, you usually aren't saving a ton of weight by going carbon until you are spending $5k plus.
I'm going to refer everyone who asks "what mountain bike should I buy?" to this
Eli Bingham post; he nailed it. For those who think that list is too spendy, I'd either go downrange on the models he listed or look at Marin. The Rift Zone and Hawk Hill (if you must go 27.5) are really well spec'd for the money you spend, and the geo is solid.
Your bigger issue will be actually finding stock. From what I understand, I wouldn't expect the bike supply chain to get back to normal until next spring at the earliest.