Such a messy relationship. A male lead who's a coward and struggles with confrontation, and a female lead who actively sabotages him when he tries, because she's afraid of him going through with it. I don't think either's on the morally good side here.
He started out dishonestly, but came to realise he just wanted to continue their relationship and throw the bet aside. At that point, his sole responsibility was to tell her his true feelings once or before she found out about the bet, and that would pretty much clear him.
She, on the other hand, started out half honestly and half manipulative, but grew more manipulative once she realised what he was trying to do. That threw any moral issues on his part to the side, since she was just as bad.
All in all, they deserve each other, and they're cute together. Just a bunch of drama created by the author to prolong the tension.