Is it a prerequisite for a sound repentance to abandon the people of sin, or its place, or its friends, by analogy with the story of the slayer of a hundred souls?

Leaving the place of sin or abandoning bad companions is not a condition for the validity of repentance, but it can be a helpful factor in achieving complete repentance. The conditions for repentance are: giving up the sin, regretting it, resolving not to return to it, and restoring rights to their owners if they are related to others. However, if leaving the place of sin becomes necessary because one is unable to abandon the sin while remaining there, then it becomes obligatory. The same applies to bad companions: abandoning them is preferable if there is no hope of reforming them, and one should associate with good people.