What would a person wisely do if he was in the lawyer's place? Why?

Human beings are social animals, and solitary confinement can be a harrowing experience. It is used as a punishment in many prisons. In most cases, a prisoner is not kept in solitary confinement for a long period of time because he is apt to go completely insane. Solitude can be easier for some people to tolerate than for others. Hamlet tells Rosencrantz and Guildenstern:

O God, I could be bounded in a nutshell and count
myself a king of infinite space . . .      II.2

Hamlet is a scholar, an intellectual, a philosopher, an introvert. If he had to spend fifteen years in solitary confinement, he would probably spend most of his time reading books and perhaps keeping a journal. A man like Laertes would go crazy in a short time and would live up to the banker's prediction concerning the lawyer in "The Bet."

"Think better of it, young man, while there is still time. To me two million is a trifle, but you are losing three or four of the best years of your life. I say three or four, because you won't stay longer." 

A man with inner resources would probably do more or less what the lawyer does. He would improve his mind. He would see that there were advantages as well as disadvantages to being alone. He could concentrate on reading and thinking. No doubt the first year would be the hardest, but he would grow accustomed to solitude in time.

The only mistake the lawyer seems to have made was that he didn't try to keep himself in good physical condition. 

At the table a man unlike ordinary people was sitting motionless. He was a skeleton with the skin drawn tight over his bones, with long curls like a woman's and a shaggy beard. His face was yellow with an earthy tint in it, his cheeks were hollow, his back long and narrow, and the hand on which his shaggy head was propped was so thin and delicate that it was dreadful to look at it. His hair was already streaked with silver, and seeing his emaciated, aged-looking face, no one would have believed that he was only forty. 

There was no reason for this. He could have invented all sorts of exercises that he could do in his room. He was evidently getting gourmet meals, so there was no problem with nutrition. It would seem that Chekhov intentionally made the lawyer's condition deteriorate so badly over the years because he wanted to make it appear easy for the banker to murder him. After all, the banker is an old man and the lawyer is still relatively young. The banker thinks to himself:

And I have only to take this half-dead man, throw him on the bed, stifle him a little with the pillow, and the most conscientious expert would find no sign of a violent death.

It could only be easy for the banker to do this if the lawyer had allowed himself to become so emaciated. There is nothing about solitary confinement, or about a life devoted to study, that should necessarily make a young man turn into "a skeleton" in fifteen years of solitude unless, for some reason, he neglects his body completely. 

A wise man should follow the lawyer's example if he were in the lawyer's place, but he should take as much care of his body as he takes of his mind. Even if he is only interested in mental activity, it is a fact that the mind performs better if a person gets good, regular exercise.