Sorry if that came across harshly - it wasn't intended to - but apart from spending a huge total of money, it's also kind of admitting defeat.
As it stands, yes, shops have the product on offer, but OP doesn't need one as she already has one. Well, she will have when the 'friend' hands hers back or replaces it.
Whilst I don't think 'friend' will hand it over willingly, OP is playing this very well and is not going to let it lie; so hopefully, she will be frightened to keep the pretence up much longer and will soon have no alternative but to hand over the item or the cost of a new one (whether it's still available for £400 or has gone back up to £500 - that's 'friend's' lookout and the longer she lies, the more it might cost her).
If/when she does hand it or a replacement, over, OP will then have two of them. Plus, to such a CF as that, if she gets wind of the purchase - or if OP tells her outright and says she now needs the cash reimbursed - it will give out the clear inferred message to her that OP must be rich and now has a replacement, so she doesn't need to worry about it any longer and the matter is 'sorted'.