Answer:
Technically the banker wins the bet.
Step-by-step explanation:
The banker and the lawyer bet on the lawyer's ability to stay in solitary confinement for 15 years. The day before the 15 year mark, the lawyer leaves his solitary confinement and forfeits the bet and the winnings. The banker would've been bankrupt had he been required to pay the lawyer and was going to kill the lawyer before he got out. However, since the lawyer realized that material goods were no use to him and he would rather work towards divine salvation, he did not want the money and thus saved his life.
Some might say that the lawyer actually comes out better than the banker in the long run. He is more at peace with himself and is better off now that he's been in solitary confinement. However, he did not technically win the bet.