Okay.. first, welcome
Second, as Paul and Carfreak said, different engines in the 350Z and 370Z. There may be parts that are interchangable, but no-one is letting on if they do know.
Third, if your friend has a 350Z, you're both in luck because the VQ engines are basically the same in both. The 350Z mods you can put on the car will not be very easily done because the Altima is FWD, and the 305Z is RWD. Intake/exhaust headers won't work, mainly because of the engine orientation and plumbing... other than that, though, you're basically good to go. Check out
[Only Registered Users Can See LinksClick Here To Register], and
[Only Registered Users Can See LinksClick Here To Register] if you're looking to do a serious engine build-up.
As far as the Exhaust, I believe
[Only Registered Users Can See LinksClick Here To Register] has good exhaust headers for both, and
[Only Registered Users Can See LinksClick Here To Register] has cat-back systems for both.
Intakes are up to you, I know there's at least 5-6 billion companies that make intakes for the 350, but SE-R you're limited to
[Only Registered Users Can See LinksClick Here To Register], Nismo/AEM and
[Only Registered Users Can See LinksClick Here To Register] for CAI, and WAI's are made by Berk, K&N (I believe) and Jim Wolf Tech has their Pop-Charger setup that's supposed to be pretty good. I've got a K&N drop-in filter, and I have no complaints.
Nismo has an S-Tune coil-over suspension for the SE-R, but I've heard mixed reviews about it. There are a few companies that make Coil-overs for the Altima. I can't really help with that because I'm going with KYB GR-2's and Eibach Pro-kits. Other than that, there are a couple companies that make Strut-Tower Braces (
[Only Registered Users Can See LinksClick Here To Register] and
[Only Registered Users Can See LinksClick Here To Register]).
The Altima isn't really a platform that lends itself to a lot of mods, but the ones that are available are pretty good bang-for-the-buck, and the car is damned fun stock (with the 6MT). Doing the main-three upgrades (intake, Exhaust, wheels/rubber) will make a noticeable difference, but if you're looking for gobs of power, be prepared to invest a bunch of time and money.