John Collins is one of Franklin's childhood besties, and his first writing buddy/intellectual companion. Early in the book, they have this sort of idyllic, long distance epistolary (a fancy way of saying letter-writing) relationship, where they write each other long tomes about big ideas and practice their argument skills. We know more about Franklin's letters than Collins's, but the point of their exchanges is how much they enjoy the process of improving their writing and stretching their brains out.
As young men, though, they drift apart. Franklin's excited to reunite with him in New York, but while they still share intellectual interests, Collins has become a gambling drunk. He relies on Franklin too much for money, while he himself is totally unreliable. Their friendship goes up in flames when Franklin tosses Collins off a boat, and Collins never pays Franklin back the money he owes him.