If you flat out tell her "no, your drunken behavior will ruin my trip" it would definitely be the end of any friendship you have. Vegas is not the place for this woman to be. Given her history though, she probably doesn't have many friends.
Tell her you got a group rate, all the arrangements have been made, it's too late now, yadayadayada, tell her anything but telling her she's a risky drunk won't be of any help to either of you. She'll (hopefully) figure that out eventually and she has to be the one to figure that out.