Have some fun. Act like you expect to win the bet. Mock your friend, tell him he doesn't have "the balls" to dress up like a slut for Halloween.
If he takes the bet you're set. If he's still resistant, just up the odds. Make your side of the bet less-and-less likely to come true until he's finally faced with a "sure fire winner".
"Not only do I think the Giant's score the next touchdown, I bet they get the next two in a row!"
"You can have blondes and brunettes, but I win if it's a redhead that walks in next."
Be confident, show some bravado and taunt your friend. Tell him you'll even buy him the wig.
Goad him into taking the bet.