A friend of mine who doesn't even play guitar needs some money and they have a guitar and want to sell it. I'd never seen this type of Gibson before so I went and had a look at it and it is a Landmark Series Nighthawk in Navajo Turquoise. Can anyone out there tell me anything else about this certain guitar as I don't really want them to get ripped off since I know they know nothing about guitars.

Also, one of the tuning pegs is loose and no longer winds the string. Has anybody else had this happen, and is it fixable or would a replacement be needed?
It's like a Les Paul that wants to be a strat. Strat scale length, slimmer neck profile, strat like selector switch, and more quaky tones. For some reason they didn't take off well.
The Nighthawks were an early 90's guitar that came in a variety of different configurations. Epiphone has a version that's close to the original Custom version (my personal favorite), and they started issuing that a couple of years ago.

The guitars have a smaller body than an LP, originally came as mahogany/mahogany (body/neck) and had three pickups: at the neck was a minihum (which kept things from getting LP muddy in that position), the middle pickup was a single coil and the bridge pickup was a full-size humbucker, but *slanted* with the treble side closer to the bridge than the bass side. The scale is a full 25.5" (rather than the 24.75" of an LP) and the five-way offered a lot of options.

Gibson eventually produced dual mini-hum and dual full-size HB versions and then discontinued the guitars. They recently reissued a double hum version (both humbuckers in a straight horizontal configuration), and that's still on the books (along with the Epi version). There's a new Nancy Wilson signature version that has the slanted humbucker in the bridge position.

Used guitars generally don't generate a lot of bucks, usually capped out (with the most desirable versions in excellent shape) at around what they sold for 20 years ago. Little/no collector value. The most desirable versions have fancy inlays (there are "cloud" inlays on the Custom) and great flame tops. The less desirable versions have dots and more or less opaque paint jobs.
Check out ebay's sold listings. Looks like they are selling between $500 - $1200 depending on the particular model and condition.