Highly likely your wife accidentally clicked on the gear limit button on the lower right side of the steering wheel. Easy to do, if she was stopped in 1st gear, and clicked on the the minus (-) symbol on the steering wheel, it would lock her in 1st gear. The trucks rev's would climp and not shift, likely why was couldn't go over 35 mph.
To fix that, you press the plus (+) button and raise the gear up, if you press it past 8th gear it will turn the feature off. Also, turning the truck off and on or switching the gear selector out of drive (i.e. to park or reverse) will also turn off the feature.
Next time, get you you truck, when you turn it on, press the minus button on the selector, it will go to your current gear, so in park it will go to 1st gear. You will see what gear its limited to (1-8) on the dash to the right of the DPNR indicators on the guages. Play with it a bit so you see how they work.