All Nikon d-slr cameras can use "FX" lenses in F-mount. There are no, as in not one,single model, that actually carries a label that says "FX" on the lens....only, and I mean ONLY, the lenses designed for small-sensor Nikoncs, have a label, which indicates format, and that format is DX, as in DX-NIKKOR ____ f/X.x, and so on.
The D40,D40x,D60, and the D3000-series and D5000-series cameras do not have in-body autofocusing motors and the screw-drive connection used for the earlier AF-series or the AF-D-series autofocusing lenses; those "small Nikon" bodies use the in-lens AF motors for focusing. But those "small Nikon" bodies can use the 50mm autofocusing lenses Nikon has made, back to the mid-1980's.
Nikon's newest lenses are AF-S, or AF-S G series models: these are what the D3000-type Nikons work most completely with.
The Nikkor 50mm autofocusing lens was made in the 1980's to the 1990's in the AF-series, and then the AF-D series, in both f/1.8 models, and f/1.4 models.
The very-newest 50mm for Nikon is the AF-S G series.
There is an f/1.4 50mm AF-S G, and also an f/1.8 AF-S G-series 50mm lens, in a new series of lenses with 20,28,35,40mm, 60mm macro, and the two 50 models, and two nice 85mm primes, EACH ONE of which has the in-lens AF-S motor, and which are available in both f/1.4 and f/1.8 aperture maximums in some lengths.
Your friend might be a bit confused it seems. Again, there are absolutely NO Nikon-made lenses that are called "FX"....there are AF and AF-D lenses, and those will mount, and take photos, but will not automatially set the focus for you on a D3000-series camera. But they WILL TAKE PHOTOS.
A 50mm lens on a Nikon D3100 or D3200 functions as a short, fast-aperture telephoto lens, with a moderately narrow angle of view.