A friend and I won a 50/50 raffle at a sporting event. He claimed it and got the 1099, how do I claim my 1/2 so he doesn’t get hit with the full amount. Do I do a W9?

Here is how to keep it simple ... have the friend compute his return only entering his 1/2 of the winnings and write down the bottom line then enter the entire winning and write down the bottom line ... the difference is what it cost him on his return so the is what you owe this friend. 

The total of the winnings was $19,000. Won’t that boost him into a higher bracket by doing the whole amount?

Maybe, possibly but it  probably won't make that big a difference ... but if you want to do it the hard way you can.  He would need to issue you a 1099-misc for your half of the winnings and file the 1096 & 1099 with the IRS  by 1/31/18  and then make a line 21 adjusting entry on his return to explain why the entire amount is not on his return then attach the explanation & the 1099 to the return he now needs to mail in.  Do you really want to go thru this bother for a couple of $$ ?  Your choice.