See further discussion under unit 92. First mate seems the best translation.

AI vs. AI. I mentioned 'second mate' to ChatGPT and was asked about context by supplying a paragraph. This was the answer:
This context is quite revealing, and it actually argues against translating styrmannen as second mate.
Notice the sequence:
Også om styrmannen, Johansen.
– Ja, styrmannen, Bertil Johansen...

The old pilot immediately recognizes "the mate" as Bertil Johansen, as though styrmannen is his identifying role. There is no qualifier such as andrestyrmannen ("second mate") or førstestyrmannen ("first mate"). If Elvestad had intended a specific rank, he could easily have written it.

I would translate the passage as:
"Is he alive?" the old pilot asked.

"Just give it time," Andreas continued, lost in his own thoughts. "You'll learn everything. About the mate, Johansen, too."
The old man's face brightened.
"Yes, the mate—Bertil Johansen. He must be about my age by now!"
or, a little more fluidly:
"You'll hear everything in due course. About Johansen, the mate, as well."

The repetition of styrmannen in the old man's reply reads naturally in English as "the mate." It suggests that this is how the man was commonly known.

About "second mate"
Your co-translator's suggestion would only be justified if there were independent evidence elsewhere in the novel that Bertil Johansen was specifically the second mate. Otherwise, translating
styrmannen, Johansen
as Second Mate Johansen
introduces a rank that the Norwegian does not express.

One further historical point: in Norwegian maritime usage around 1900, styrmann often functioned as a generic title for a navigating officer. Which officer it denoted depended on the size of the vessel. On a smaller coastal steamer or sailing vessel there might be only one styrmann besides the captain, in which case the mate is exactly the right English equivalent.

So, unless later chapters explicitly identify Bertil Johansen as andrestyrmann, I would stay with the mate. It is both faithful to the Norwegian and idiomatic in English.

Thanks for the education, David.

As "styrmannen" continues, I am translating it according to my ol' hard cover dictionary: Second mate
Here's why. This is from Google in AI mode:
No, the helmsman is not the same as the first mate or second mate.
Here is the breakdown of the different roles:
The Helmsman Definition: The person who physically steers the ship.
Role: They control the helm (the wheel or tiller) to maintain the course.
Rank: This is a duty, not a high rank. Any qualified crew member, able seaman, or officer can act as the helmsman.
The First Mate (Chief Mate)
Definition: The second-in-command of the ship.
Role: They manage deck operations, cargo, and crew schedules.
Rank: High-ranking officer who takes command if the captain is incapacitated.
The Second Mate Definition: The ship's navigation officer.
Role: They plot the ship's course, maintain charts, and manage navigation equipment.
Rank: The third-in-command officer, subordinate to the first mate.
Summary of Difference
The mates are high-ranking officers who make decisions and give commands. The helmsman is the person who executes those commands by physically steering the wheel.

"First officer" may be correct, but perhaps more often we might find "helmsman" or "chief mate". This according to the Glosbe online dictionary - https://glosbe.com/nb/en/styrmann
My handy McKay's Modern Dictionary has it as "second mate".