Sadly, many dealers don't do much water testing after rigging a boat. If the motor is hung, the rig cranks, and the whole thing runs down the lake, they're going to call it good. Tinkering with setup is pretty much a given. I wouldn't be too upset about it. It's part of buying a new boat.

First, and just to make sure nothing funky is happening, I'd get under the boat and inspect for anything odd, like bows, hooks, blemishes or excess weld material on the bottom. This doesn't sound like the problem, but weird stuff happens when it's there. I would be happy with 6,000 rpms on the motor side and I would NOT want that to change. Lugging is way worse for an outboard than the high side of manufacturer recommended rpm ranges. You want it to hit the top of the RPM range when its lightly loaded and all he's going to do from here out is add more stuff, so you can pretty much count on that number dropping a few rpms anyway. Trim can take a lot of the porpoising out and I don't know how much motor height (not trim) affects it, as you want as much motor out of the water as possible. That being said, he could have a prop that is providing too much bow lift and causing a lot of the porpoising, in which case, a prop change could make all the difference in the world, however, I don't think a pitch change should be the first stop. I mean, you could fix it with a pitch change, but that's because the boat is going slower. Your dealer should be happy to provide your buddy with a couple of different props that he can try out to get what he wants. After that, it's time to start shifting weight around until he hits a sweet spot. If he's running a big trolling motor, he may want to find a way to move his TM batteries further forward. If none of those things work, he may want to think about trim tabs, either manual or fixed.