I just checked the signup promos at Expressbet, NYRA, and DRFBets, and the best bonus I found is at DRFBets.com:
http://drfbets.com/VIP
You get a $300 bonus paid out $100 at a time in the first 3 months, as long as you bet $100 that month.
The other two are $200 or less.
Your buddy could bet $50 on the Derby, $50 on the Preakness, $100 on the Metropolitan, and $100 on the Belmont, and still be guaranteed of having at least $500 in his account.