I wouldn’t say its the phone company’s fault directly but did they even try to contact your friend in any way to let them know that their bill is now 2, 3, 4 or even 5 times bigger than usual?
I mean, why should they? Most phone companies allow you to set budgets, I do not see this as an issue with the phone company in this scenario at all. As other people have said, parents should be monitoring their kids spending (if at all), most stores have an option to require the password - I do not see why this is not enough.
In this case it's a roaming surcharge not the kid spending money in the store.