Sonar on a Mac

They're still going to purchase a Windows OS
http://www.parallels.com/products/desktop/
 
Haven't tried it for audio apps but seams to work for about everything else.

On a Mac he already have GarageBand which is pretty good. Logic Pro is a really good and affordable for what you get. But Bootcamp is the way to go if your friend have to use Sonar on a Mac - and as other have stated, he will need a copy of windows. Running it through Parallels or other virtualization can be a cumbersome experience depending on what type of hardware you are using.
 
Sonar works really well on my Macs in Windows through Bootcamp, but it will drain the battery of a laptop really quick. My iMac also runs a bit hot but no biggie. The stability is surprisingly good, and although I don't use Sonar as much anymore, it seems to run better than it ever did on my WinPC's. I don't think your friend will run into any problems, but I would check out some of the DAWs already available on Macs

I second the advice that he should start by trying Garageband.

It's aimed at DAW beginners but can still produce reasonable results, although the tools and options available are limited. Which is probably no bad thing for a DAW beginner. It's installed on modern Macs by default and can be downloaded from the app store for free if it isn't already there or he's deleted it in the past. It also integrates well with Logic, which can open Garageband projects and even defaults on installation to resemble Garageband and with all Logic's advanced stuff hidden,

As for noise, etc. that'll be the same problem no matter which DAW. Built-in mics tend to be a little noisy to very noisy on any computer, and guitars tend to be noisy... A decent inexpensive interface will help, and so long as it supports Core Audio (almost all do) it should give low latency out of the box - no need to tune OS X, no need for ASIO drivers and all the other stuff that we go through with Windows. If he gets an interface with ASIO drivers he can always use it if he decides to switch to Windows and Sonar at some point.

If he had a PC I'd suggest without any hesitation at all that he tries a trial copy of Sonar. Since he doesn't have a Windows PC his zero-cost option is to try Garageband and see how he gets on. All DAWs (with the exception of Live) have a lot in common in terms of interface, what they do etc. and experience on one usually translates to others. Audio recording and engineering principles are the same no matter which software you use.

The one thing I think is a downside to Logic is that many of the supplied plugins aren't as good as Sonar's. The synths are good, the compressors are OK and the eq is well, a standard issue one of the type found in all DAWs. The "effects" end of things, such as modulation plugins and delays, is better covered by Sonar. Third party plugs can fill all the gaps of course, but add substantially to the cost if you're not happy with those bundled with Logic. How Logic handles cpu threading when monitoring via Logic can be a pest as well. Amplitube alone almost brings my i7 MacBook Pro to its knees because Logic pushes it all through one core which hits 100% unless latency is set rather high. My sig PC hardly notices it running in Sonar.

Edited to add a PS. If he's using a power PC Mac it is going to be seriously old, in computer terms it's an antique. I know a couple of people who still use them amd swear by them, but it's more to do with their personal "religious" opposition to changing how anything was done over a decade ago than anything sensible. Like they think the web was perverted from the oroginal intent and ruined when pictures arrived... Let's just say they're a bit weird.

That iMac's a little underpowered perhaps, but should run Garageband and even Logic so long as it's not too heavily loaded. The 4GB RAM is possibly the weak spot, though OS X is pretty light on system resources as modern operating systems go.

One thing he probably would benefit from is an external 7200rpm drive (with its own power supply) to spool audio to. The hard drive built into a lot of iMacs is a 5400rpm one and spooling multitrack audio to and from it while it's also used for paging and is the system drive might be a bit much for it to cope with. Apple hybrid drives also have a bit of a question mark regarding using them for audio spooling. If it has an Apple-fitted SSD that should cope OK to start with, but an external drive is still the better way to go.