I must agree with bastiat and the others. A drum-only shop would need to be located in a major city (or close suburb of one) that has a ton of music industry presence. Meaning: NY, LA, Bay Area, Nashville, Atlanta, Miami, Seattle, Austin. Even then, most of your sales would be lower-end stuff, with occasional sales of high-end kits to touring/recording bands. And even then, a drum-only shop would probably only work if you were a famous, retired drummer who could pull in the investment dollars and had the connections to make this happen in an expensive major city.
Out in Podunk, NE, you'd be lucky to sell 100 kits a year, with 90+% of them being entry-level kits. You'd need a ton of money to lease a big store and keep it filled with a variety of inventory, because if you don't already have it in stock, most kids will just go to the Internet and order it to get exactly what they want. How many sticks, stick bags, drum keys, and heads do you need to sell a month to cover business costs? Have you checked to see what advertising costs?
Finally, when I look at the big music stores in the Bay Area, the guitar rooms are HUGE, with HUNDREDS of guitars and dozens of brands of amps. The keyboard section has all 9 major brands, with 3-6 models each, and lots of rack-mount accessories. The drum rooms are small, with 6-10 kits total. Reason? There is relatively low turnover for drum kits, and much higher demand for drummers than for guitarists and keyboardists. And few drummers have more than one kit, whereas guitarists and keyboardists usually have collections.
Bottom line: your friend needs to put the pipe down and learn a bit about the industry before he puts himself into tens or hundreds of thousands of dollars in debt.
-Troy