If you live in Denver I assume you have a car. There is a lot of interesting mountains west of Denver on the way to SLC that you really might like to see and know about. If you fly to SLC and rent a car, how does that work out for being cheaper than you just driving all the way?

Not sure I understand why you would want to go to SLC to go to Yellowstone. It is not on the way. SLC may be closer since it is further west but you still have to go north a long ways. It not the most interesting drive between Denver and Yellowstone - wide open Wyoming but it improves the last hundred miles or so. Unless I am missing a big something, just drive from Denver. You might want to check a map of the area.

Actually the drive from SLC to Yellowstone is quite beautiful in spots.
I guess the other thing to consider if he's going to fly would be to fly to Jackson and rent the car there. I'm really not "getting" this.

SLC is 2.5 hours closer but you'd spend at least that much time driving to DIA, waiting for your flight, flying and then gathering your luggage and renting a car in SLC. When all is said and done, you'd probably spend more total time travelling if you fly to SLC rather than drive directly from Denver to Yellowstone.

You also need to look at the opening dates of facilities in Yellowstone, there are lots of things that aren't open yet at that time of year.

http://www.nps.gov/yell/planyourvisit/open_closedates.htm

There's a 3.5 hour difference each way...hardly seems worth the expense of the flight and rental car imo.

Coming into the Tetons through the Wind River area is scenic.

Go one way and come back the other. The entry from the south through Jackson is beautiful--Jordan canyon or such?