It's still working but the screen is totally cracked.
My car's still working - the engine has totally blown up but four people can still sit in it whenever they want to. If we're talking a small scratch in the top corner, that's one thing; but if the screen is totally cracked, then it isn't working. What use is it if the phone is able to render the information perfectly well if the owner can't actually see it properly?
I can't afford half's either tbh.
It's about £80 for a new screen.
That's £40 I could use for other things.
So which is it: can you not afford it or would you just rather not have to pay for the damage you caused? Should insurance companies work on the same basis, having taken your premiums - if you singe your carpet, they'll pay for a new one, but if your whole house burns down after being hit by lightning, you're on your own there because it's too much money for them to want to pay - money that they'd rather spend on other things?
Have you thought that, just maybe, SHE can't afford to pay £40 or £80 to repair something that was smashed owing to the carelessness of a 'friend' - or maybe she could afford it but still thinks that it's £40/£80 that she could spend on other things?
Do you have a similar-spec phone handset? Why don't you ask if she'd like to swap with you, with no money needing to change hands? She shouldn't have to live with a smashed screen that she wasn't responsible for, but if "it's still working" then you'll be absolutely fine with it, won't you?