Future in the past: would come or came?

The sentence below is from the short story Death of the Island Doctor by Gene Wolfe. The two students ("they" in the sentence) are discussing what they will tell the professor ("Dr. Insula") when they come to his seminar again next week:

And when they had cooled the beer in the leaf-brown river, and eaten
the sandwiches they had brought, they paddled back to the spot where
they had parked the Toyota, debating how they could tell Dr. Insula he
had been mistaken about the island when they came next week—how they
could tell him there was no magic there.

The clause "when they came next week" is in the future from their perspective, which is the past. So shouldn’t the normative way of writing it be "when they would come next week"? Is the sentence correct/idiomatic/strange as written?