Won the lottery but friend is trying to steal the money?

I went into the store to buy scratch offs, i buy a bunch using MY MONEY. My friend is in the car and i ask him to help me scratch off the tickets, as were scratching were losing but then he scratched one and won 100,000. I check the ticket and almost crap myself. I cant believe it and i tell him im rich oh my God. He then snatches the ticket from me and says "its my ticket i scratchd it" I said no its not i bought the ticket its mine. He starts saying how no its his so i pull him out of my car, we get in a fight and i manage to get the ticket back. he then starts screaming its his ticket. As i tell him no its not its mine he asks to split the money because he scratchet it. What do i do people? WHY IS HE ACTING LIKE A PRICK