If your brother already has an f/2.8 lens, then an updated body might give more benefit. If he's shooting with an f/4 lens, then harder to say. Edit: from your comment, he does have f2.8 glass, so f/2 isn't going to give him significant benefit.
There is no clear upgrade path for the D300. A D400 has been rumoured for some time, but there seems to be a lot of doubt as to whether Nikon will release a D300 follow-on.
The D300 is 13MP, shoots at 6 frames/sec and has 51pt AF.
The only true DX camera that is in the running is the D7000, unless he would consider the D600 or D800, which can shoot in DX modes.
D7000 - DX. 16MP, 6 fps, 39pt AF
D7100 - DX - 24MP, 6 fps, 51pt AF
D600 - FX, 24MP, 5.5 fps, 39pt AF
D800 - FX, 36MP, 5 fps*, 51pt AF ( * or 5.5 fps in DX mode, 6 fps with battery grip)
Both the D600 and D800 have DX modes which will give him the extra 1.5x reach that he's getting with the D300.
I'm not sure the D7000 is enough of a step up to warrant upgrading. Better low light performance, but not sure build quality, AF performance as good as D300.
I think the D800 would be fantastic if budget allows. Significantly better low light performance.
Nikon may announce a D400, and should announce a D7000 update, sometime this year. If he wants to stick with DX, and the D800 would blow his budget, then I'd suggest he waits to see what they announce later this year.