You'll need to pull the codes to be sure but there are a number of possibilities here, one being the UVCH which were mentioned above. If it is the UVC's then you should have a pronounced miss along with the power loss.
If the power loss is just a "seat of the pants truck has no umphh" loss then possibly the EBPV is closing and restricting exhaust flow which should make a loud whooshing noise This is usually an intermittent type of problem though, so some times when warm she should run just fine and then fall on her face when the valve closes.
Or a third possibility is that he needs to rebuild the IPR valve and this one is the one getting my bet. Truck runs fine, idles fine when cold but when it warms up it has no power and should also display a lope or more likely a surge in the idle.