Hi Heather,
welcome from another new member of this forum. I assume that you're referring to an used bike becouse it isn't still sold.
First of all, he doesn't have to change his old GS500, he can keep it and add a "new" 2nd hand Fazer600.
If he's fascinated by a bit 'old models, the former Fazer 600, the FZS600, IMHO is better than newer.
The way in which the power is supplied by her engine and the fuel consumption is better than the Fazer/FZ6 model... The suspensions are better, the brakes are better (except in the "S2" model, where are just the same). Moreover, the price is very interesting, in Italy you can take a 2003 good condition FZS600 for around 2000 euros, I think in UK the same.
What do you have to check in a 2nd hand bike ?
Look for scratches (symptom of a slip), brake discs without jagged wear (too many kilometers), transission chain and related cogwheels, the front fork legs, the brake lever and the clutch lever which must not be bent, irregularities of the engine noise, no smoke at the exhaust. And of course the tyres.
Here she is:
Let us know how it turned out.
Best wishes for a good buy