Hi Mark,
I see your brother not only made the right choice in buying a Fazer FZS 600 (Not quite sure why he bought it as he has done no mileage on it) but he also bought one in the best colour.
Now my Fazer FZS600 is exactly the same year Foxeye 2003 in the same colour, I purchased mine about 4 maybe 5 years ago at the beginning of the year after Christmas
(A good time to buy as people are always short of dosh after Christmas)
My bike had something like 28,000 miles on it and in lovely much loved condition, I paid a garage £1995-00 for mine, but bearing in mind that was possibly 5 years ago, I also got 3 month parts and Labour warranty with it.
I would guess that the first thing anyone would do with your Bros bike is to put new tyres on it as they will have perished to some degree, standing for about 4 years, drain the tank and refill with fresh fuel and possibly a carb clean (Not not necessarily).
With the time of year (A bit cold for biking) and the fact that many will not have a lot of dosh to hand, being Christmas, I would have guessed at around £1700 - £1900. obviously dependant on inspection.
Nice bike, why don't you buy it off your Bro offer him £1500 and get out on it and have some fun.
PS: What is the other bike in the garage? Could it be an R6?