1. of course he can go to the police. it's his work. it's his - dunno how this is called in english, but in german it would be - "geistiges Eigentum".
the only strange thing is that he didn't write his name in it. maybe this helps you when he goes to the police. anyway, i dont think this will have much effect - how shall he prove that
you ripped it? maybe
he ripped it from you?
this is very hard to prove and i dont think he is able to do that.
2. just writing "ripped by" would have no effect. its like stealing a car and writing "stolen from xxx" on the window
if you want to use his script then you have to talk to him. you will have to agree to his conditions. if he disallows you to use the script then there is little you can do. (but as mentioned above: he has to prove that
he wrote it before he can disallow anything).
my opinion. no guarantee.
edit: this is how it looks like in germany. dunno how this is handled in other countries.