Hola AA

I think it's necessary to look at the text around that sentence to see more clearly the use of the preterite as opposed to the imperfect.

If you go back to the text, you notice how in the first sentences the narrator is using mainly the imperfect because they're telling us some background information: the taxi driver was having a lonely life, he was having problems with alcohol... And then the dynamic changes and starts telling us about things that happened form a specific point: yesterday... and continues expressing things that are seen now as completed actions.

One of these actions is that their friendship grew. If you take that bit out of that context and see the phrase "it grew over time" you think of a progressive action using the imperfect, but within that context the narrator is seeing this as something that happened and he sees the end of it (not really the progress in time) - it is as if we said "y después de unos meses, su amistad creció"  (and after some time/some months their friendship grew- we see it as a result "past" that time, not "during" that time. 

To convey this "action completed" effect we use the preterite. 

I hope this showed some light (it's a tricky one though...)

Saludos cordiales

Inma