Take the fact that you spend money on this girl out of the equation. This isn't tit for tat, and it wouldn't let you off the hook if one of your kids really did break her computer. When you choose to feed her, take her on trips, and treat her as your own child, you must own those decisions and not let that factor into this situation with the broken computer.
From your description of events, there is no way to tell exactly what happened to this computer. This girl could have dropped it herself one too many times, and now, it's just dying. Any number of things could have happened to it at your house, her own home, or elsewhere. She's 15 years old, and if her parents have decided she's old enough to have and be responsible for an expensive piece of technology and to take it out of her own home, then that is their decision to own, one for which they must take responsibility.
Now, if one of your children did, in fact, do something to the computer, and everyone saw it, that's a different story. If one of your children did break the computer, then you should do the right thing and replace it.
Going forward, you should have some set rules about technology in your home, and you should be clear about those rules with other parents. You should ensure that your daughter's friends have a safe place to put their tech gadgets out of the way and that they do so. Be clear that if things are left out or left on a counter, that you cannot be responsible for them.
In this case, just from what you've shared, I think that the parents of your daughter's friend should take responsibility and repair or replace their daughter's laptop.