Since I was also confused about the bet I went back. The important parts are between chapters 13 and 14.2.
The bet was between Ataru and some newbie harrassing D-rank party that said Ataru and Caro as F-ranks were only after the participation reward.
Ataru and them bet who'd kill more monsters. The winner would get the loser's participation and monster kill rewards plus 20 gold from the elf.
When the battle was going on and they saw Caro wipe the floor with the monsters they decided to catch her and tie her to a tree, taking her out of combat (and accepting her probably getting killed by monsters), however when they approached her Ataru took out their weapons and the rope they readied and they fell prey to some lizardmen.
I think that's the last we saw of them. Didn't read any further, because you know, didn't want to go through the entirety of chapter 14 again.