A number of questions spring to mind.
1. Is this the first Alfa he has had. The Alfa 156 gained an unfair reputation for customer satisafaction because a lot of people that bought one previously owned a BMW or Volvo and to be truthfull the build quality is not the same.
2. Are you prepared for the car to spend time at the garage? Most Alfa's enjoy a healthy relationship with the local dealers built up over the many hours they spend there.
3. Are you a driver? The Alfa is a drivers car and needs to be driven hard, not pootled about in.
4. Are you prepared to fork out cash at the most inconvenient times. The Alfa knows when it's the wifes birthday, Christmas etc and will require you to spend your money on it at that time, its a sort of jealousy thing!
Above all though the V6 156 is a cracking car and well worth the money second hand, but you need to remember that it not built like a BMW and will require repair, however nobody second glances a BMW but an Alfa turns heads, whatever one it is.
I had a 156 V6 24V SP3 with full body kit, lowered etc with a one off Momo embossed leather interior (most were Alfa embossed) and I drove it very hard for 60,000 miles. It never actually stranded me but it had its faults, especially electrical and I had a lot of parts replaced under warranty. Basically though the car was abused to the full and I loved every minute of it.
Servicing is relatively cheap (cheaper than my HGT!) but if it needs a part you may have to wait for it to arrive, which was something I found annoying. Front tyres get shredded quickly, especially on the SP3 beacuse of the lowered suspension.
If the clock was rolled back to the day I had to decide to buy it or not, the decision would be yes again without a doubt, I loved it, it is the best saloon Alfa ever.
If you buy one and don't like it, advertise it and it will sell very quickly as there is a demand for them and the second hand value is cheap becuase people are still afraid of them.
Should you wish to email me any direct questions or further advice on the 156 V6 please feel free, but if it hasn't got a CD changer I will feel the need to try to sell one to you as I have one sitting beside me!
All the best
Nige