Good story.
Sometimes you gotta take a leap of faith and hope a kind gesture doesn't backfire on you. 'Cause sometimes, it does...as the OP may have cause for regret.
Hopefully, the friend will make good.
I remember that the kid had asked me where I thought the bus was (at what point of the journey) and had discussed distances and estimated times of arrival with me; at that time, I travelled a lot on that route, and had a very good idea indeed of the entire journey, and so was pretty accurately able to estimate times, and distances between any two points on the trip.
More to the point, I remember having been very impressed by his manners; he was a polite, decent, responsible kid, the sort any parent would have been proud of.
Thus, on the general issue, I honestly think that basic decency and a sort of cost-benefit analysis - but measured on an personal or psychological scale, rather than a materialistic one - are what is called for here.
To me, measuring on such a scale, ensuring that someone could call a taxi (basically enabling them to get home safely) is of a far higher order of need than keeping a phone pristine. Of course, I give them the phone, or make the call for them.
(On the topic of phones, and iPods: Mine are always encased in leather, and that includes the antique Nokia that I am currently using; I don't 'do' what some on these threads describe as 'naked' phones).
Now, I will readily concede that if someone shows a consistent pattern of behaviour whereby they are casual, cavalier, and careless with my possessions, well, yes, the degree of friendship might well suffer, as what you are seeing is a lack of respect.
But, as for accidents? They happen.
Mention it to the friend, and see what transpires.
Several years ago, I was at a formal reception when a retired colonel with whom I was working, and with whom I was engaged in a very serious conversation, accidentally spilled a glass of red wine on my (expensive bespoke) jacket. He was horrified, and profusely apologetic, and asked to make good the cost - irrespective of what it was, especially if it required specialist cleaning - of the cleaning when we both returned to our respective countries, insisting that I take his contact details.
Anyway, the jacket was made of good tweed, a robust material, and I figured that water and a decent rinse would do the trick (it did) and I wouldn't hear of letting my colleague pay. However, I will admit that my already considerable respect for him was reinforced by the fact that he offered (genuinely - he raised the topic subsequently) to pay for this.
