Word for a person who is challenged to a bet [duplicate]
This is either the exact word you're looking for or I'm very close to finding it:
stakeholder (n.)
1. (In gambling) an independent party with whom each of those who make a wager deposits the money or counters wagered.
This definition doesn't fit your situation exactly, but you can see how it applies well. The stakes (4 pints) are held by him and his actions.
After a little more digging I came across another term that made me feel slightly disappointed in myself for not knowing it already:
taker (n.)
2. A person who takes a bet or accepts an offer or challenge