yr gonna struggle to get a MTb that doesnt disintergrate that will adequately cover all those disciplines.
The DH you see in all the media looks cool - but requires such a specialist bike that you'll not be aable to dirt jump or ride XC on it. If yr mate is a beginner get a hardtail, pref with dics brakes IMO, and get out over the moors - build experience and fitness with the odd big rocky descent to get the gravity hit in.
If yr mate then fancies true DH he'll need another bike.