This is a question about a specific use of the simple past after als. It's different from the more general question of when to use the simple past and the present perfect, which has been discussed already in previous posts.
I have a friend from Hamburg who seems to prefer the simple past after als with verbs that she would otherwise use in the present perfect, such as:
Ich war gerade angekommen, als du mich sahst.
German is not my native language, but it's very jarring to me to hear the simple past in the 2nd person for verbs other than sein, haben, werden, and the modals, even when talking to someone from Northern Germany. I probably would say,
Ich war gerade angekommen, als du mich gesehen hast.
but my friend says sahst sounds better to her. And this happens in very informal spoken conversations with verbs that I rarely hear in the simple past (e.g., ... als das Telefon klingelte.)
Is there some general grammatical rule about verb tenses after als, or is this just a regional or age-based style preference? (My friend is elderly.)