It's a risk
I bought my '04 FZ6 Fazer from a guy on e-bay. It was a category D write-off with a dent on the tank and a hefty scratch on the frame. The reason I took the risk was that it had 7700 miles on the clock and came with a month guarantee and the guy had consistent good feedback on e-bay.
Fact is that the guy had got the bike in an auction as a write off and just put an MOT on it then put it on ebay. It didn't need any repairs and I've left the dent and the scratch as it's parked on the street so who cares if it looks a bit duffed-up.
My advice is be cautious, but don't dismiss a good deal if it comes your way. Problem with a CBR600 is that with that fairing you can't see the whole bike easily. I could get a really good look at the bike I bought as the engine was exposed and you could tell it had been looked after - no furring on the bolts, engine pristine and down pipes shiny.
However, if your mate can try out the bike, has friends who can give an opinion and gets a kosher guarantee - even a couple of weeks - then I'd say it's worth considering. Mind you, any deal that seems to good to be true probably is.
Best of luck,