Did the bag come with any paper to indicate it is authentic?
I'm trying to read between the lines on this this one, but when you say he got a 65% discount on the bag I am assuming we are not talking about a retail shop he works in.
That sort of discount would be coming from the manufacturer themselves, because I've certainly never seen any hand bag with a 65% retail mark-up.
So, to assume a little further, if he works where the bags are produced he should most definitely be able to get paper work tom prove its authenticity I would think.
Proving authenticity is probably a separate issue, it's really no one's business what he paid for the bag, so as bevsey has suggested, just black the price out.
Personally, I wouldn't be telling anyone that I got it at a discount. You start talking like that and you start to devalue the item in some customers eyes. When you say I only paid 210, but it's worth 600, some customers will adopt the attitude well if you only paid 210, why are you trying to rip me off, no matter what the actual value of the product really is.
Give them the full retail value, the price they would pay if they walked into a retail outlet and purchased the bag themselves. Then if you can back that up with some proof of authentication, you have the perfect storm!