...He won't be able to use an audio encoder if the songs he has purchased are in M4P format (protected AAC file).
I agree with the burn to CD and rip method. It's the only easy way of getting M4P to MP3. Yes, audio quality may degrade using this method, but it's usually not noticeable, and you can help eliminate audio quality degradation by ripping at higher bitrates than the original (for example, burning a 128Kbps M4P to CD, then ripping the track to MP3 at 192Kbps, so that more samples of the audio are being taken).
There is a free program available which can decrypt protected AAC files:
you always lose quality when converting to another format, when you burn your actaully converting to a wav format, then when you rip it you re-encode it to mp3. even though it says 192, the quality will not be the same...
its probably not audible but still..
oh and also itunes uses AAC and MP4 which are "loseless' codecs, MP3 is a loseless codec.