Dad's Bike

In addition to gardening, I would like to take some time this summer to work on my bicycle mechanic skills. I've done a number of repairs on the bikes I've owned, but they are all fixies and road bikes. My dad has a department store mountain bike (Schwinn, High Timber). The bottom sprocket on his cassette is completely shredded, and the teeth on some of the others are worn down (this bike is a year old). In theory this should be a simple repair. Buy a new cassette, slip it on, and tighten it. In practice it's much more difficult, because Schwinn bikes are using this off-brand component, DNP. The DNP freehub has three tiers, whereas name-brand freehubs have one diameter from base to tip, so it's not as simple as just ordering a new cassette.
I might have to replace the entire freehub, which I've never done before. There are some tutorials online, so I'm going to give it a shot, but the problem with department store bikes is that there cheapie components don't always conform to industry standards, so the online help may not apply.