You'll find plenty of messages on this topic in the forum. For example, check out "ways to save money on a roadtrip" in the subsection "summer road trips" and "cheap and free places to sleep" under the "Gear up" subsection. You also can tell your friend that adventure is a big part of the trip, and no adventure means no fun!:-) You'll see, soon you'll be eating dill pickles chips and chinese food for breakfast and you'll like it!:-)
Seriously, if you're ok with tent camping, you'll find many free and cheap opportunities along the way and you could enjoy a hot shower every morning for a minimal fee. If not, you can stick with the Motel 6 chain, they usually have the cheapest clean rooms, or ask locals about small motels, don't forget to pick your free Hotels/Motels rebate coupon guide in every State's welcome center.
Usually, the best way to save on food, is to bring an ice chest. That way, your friend can't complaint about what they're eating because they're the ones who chose what's in the fridge!:-) Unless you like to eat only cold stuff, you should consider buying one of those butane camp stoves (about 28$ in truck stops), it works well even when it's windy, it's compact, inexpensive (butane cans last a couple of days and you can find them for 1$ each in dollar stores), and the possibilities are endless : steak, pizzas, soup, pastas, seafood, eggs, chili, vegetables, coffee, etc. Once in a while, you just have to take a few minutes to fill up your ice chest at a grocery store or in a Wal-Mart superstore.
Have a nice trip and enjoy every minute!