My young but 6'-1" tall ridding buddy asked if I could true up his front wheel (29" wheel w/2.6 inch wide mountain tires) on his Schwinn Axum mountain bike. We lifted the bike up and spun the wheel around. It seemed very true (maybe 1 or 2mm lateral) considering the rough treatment he has shown it..He said it was looked more out of true while he was riding it. I'm wondering if this sounds familiar to anyone?

Pull the rim strip and check for cracks between the spoke holes on the inside of the rim. Wide mountain bike tires can push outward on the rim and cause it to crack. The result is a rim that seems sound without load but can be out of true when the rider sits on it.

You might also check all of the spokes to ensure that you don’t have an overly loose one (or 2 or 10)

It's hard to see a 1-2 mm wobble as you're riding, especially as you're rolling on a mountain. You might look carefully at the tires; a bulge in a sidewall is very visible (and often ready to blow out).

Possibly, as mentioned above, rider is just seeing a wobble in the tire. Tires are seldom perfect, and damage to the casing will cause major wobble - usually enough to 'feel' the wobble.

As others said cosmetic flaws in the tire will easily make you believe a wheel isn't true. Sometimes the tread strip doesn't get laid straight and in the proper line before they vulcanize it on or what ever they do nowadays. Sometimes incorrect seating of the tire bead will make it lean to one side more in some places.

But I'd suppose it's possible that if the tensions on the spokes are way wonky then that might be part of the issue too when weight is put on the bike. I use to true my own wheels, but eventually realized that I could have a perfectly true wheel and some spokes be loose and others too tight. Since I didn't have anything to measure the spoke tension and didn't want to have more tools laying around for occasional use, I decided to let a bike shop handle my wheels when I need such. And I found out they don't charge much to do a really good job on them. So more time for me to do other stuff.

I'll offer a reason why a wheel might be thought of as out of true when transferred to another bike that has nothing to do with spokes or tires. The hub bearing adjustment might have shifted during the transfer. Especially with a nutted axle the lock nuts and cones can spin on the axle and thus change the bearing adjustment. One might think that the rider would recognize the flopping about wheel as a bearing issue and not a rim or tire trueness issue. But after working in a LBS one will know better than to trust a lay person's description, that hasn't been confirmed by you. Andy (not trying to say this is what's going on. Just adding info to the thread)

With a 2.6" tire, I'm not sure if you can see the rim well enough when riding?
Give the spokes a "pluck" test. They should "twang", not "thunk".
Squeeze adjacent pairs together firmly and see if any pair gives unusual results compared to the other pairs.

Is the tire seated straight?

Look at the hub.