@notlucreziaborgia
That's alright. We can't agree on everything :)
Personal story: a friend stayed over for a few days (so in this case, I was arguably doing him a favour and not the reverse) and forgot to give me my key back. He stupidly posted it back to my address without us discussing it and then packaged it badly and it got lost... or intercepted. He should have at least not posted it to the same address as the door it fitted, but he didn't think. He was, essentially, careless; not deliberately careless because it wasn't his key but accidentally careless because he has a PhD and isn't always 100% practical.
I didn't charge him for the (inexpensive) lock cylinder that "had to" be changed because 1 he is my friend, and because 2 the cost of this would neither have made me rich nor poor. Also because it was my key and I entrusted it to him without making it absolutely clear how he was to handle it in every circumstance. I'd merely assumed a certain level of common sense, like follow Royal Mail guidelines, but he didn't have that level of common sense. He was very apologetic, and probably would have paid for reasonable costs, but I didn't want him to.
I don't think either point of view is 'wrong' and you're right from your perspective. 'Someone has to lose out and why should it be the friend?' is a valid question.
My perspective is when a friend does you what seems like a small favour by bringing you something and loses it, not because she doesn't care about it, but due to momentary inattention (or it simply got nicked - and I wouldn't have had someone else's bag on my lap either, as then it gets dropped and something gets broken and then there's nail polish all over the Japanese denim jeans or whatever), she has done the favour to the best of her ability. And if her reward for being a friend and carrying the bag is now she has to pay £5 a week for the next two years, the favour was a big ask, not a small one. And that's why I feel that the person who should be offering to lose out should be the friend.
So no, no way would I expect her to pay the £600 if I were the friend. We may still not agree, but I hope you see why I feel how I do.
EDITED to clarify a badly-worded sentence.