The Henry’s Fork of the Snake requires a PhD in entomology and fish psychology. It is said that when you become a master guide on that river through all the seasons, you can guide absolutely anywhere. That river will kick eastcoast fishermen in the rear if they have no guide. (Speaking from experience).
The river through the canyon, upstream of the Harriman Ranch or the Railroad ranch, would be a bit much for people without appreciable experience on the oars. There are guides that go through there every day with catarafts and drift boats. I don’t know that anyone rents boats around Last Chance, but would not recommend that they learn it all by themselves. Downstream from the RR ranch there are some spectaculr falls. Not a place you want to take a drift boat or cataraft.
Any time you have a boat, you have an advantage, because you can cover so much more water. I recommend fishing that river from a boat, and well as most rivers that are too broad and deep to wade. Mike Lawson’s shop there at Last Chance had a lot of good guides over the years, and there are others. Lawson and Gary LaFontaine were two fellows pretty well known for their insight into the Henry’s Fork. I am not sure if Lawson has consummated a sale of it yet.
From Last Chance, you are close to a lot of things: Yellowstone, Tetons, all of the famous SW MT rivers like Madison, Gallatin and tributaries; the Snake around Jackson, ; The South Fork of the Snake which is great to drift. In that area, a 2 hour drive opens up endless opportunities.
The main stem of the Snake around Jackson (Jackson Hole) is a beatiful drift and a nice view of the Tetons. It is mostly cuts, and they are more easily fooled than other trout.
The South Fork is definitely a boat fishing river, and is a great destination.
There are also smaller rivers like the Teton.
July is likely to be busy. You need to watch the water situation, because snowfall on the ranges that proivde those rivers was not good late in the winter. I am not sure if they got enough to make up in early spring. Benjo and others in the area will prolly be along to elucidate on the snowpack.
If the area has low water and closings and restrictions, you may want to consider the San Juan or the rivers of So. Colorado. They got lots of snow, and should be in relatively good condition. In fact, the runoff may last longer, necessitating later trip dates. You need a guide on the San Juan too, it is technical with small flies, and the river fishes differently every season. With a couple of days guided, one with all the basic skills should be able to catch fish. You shoud be forewarned, fishing that river is highly addictive. They may want to move to New Mexico when they return to KY.