Translation:My friend was in New York yesterday.
Why is 了 not used with 在?
I think of it this way: I don't say, "I was atted New York yesterday." In Chinese "to be" verbs remain the same regardless of the time frame. 在 is a verb that includes "to be"in its meaning along with "at." So I literally say with 在, "Today I am at; tomorrow I am at; and yesterday I am at."
So I read a fluentu article recently, which basically said, if I am not mistaken, that Chinese has no tenses, so I would say that they just used the the day to validate when the event occurred. Hopefully I am understanding what both you and the article I mentioned are saying. https://www.fluentu.com/blog/chinese/2013/10/30/chinese-sentence-structure/. Thus, I would say that your consideration of zai as a more or less perpetually "present tense" verb would be correct, but I would think of zai as "in" or "at" in this context, but then I do not know why duolingo would be teaching us le and zai to go before or after words...
I live in upstate NY, and when i travel to different parts of the country when people say "New York" they could be referring to the city or the state depending on where i am and the context of the conversation.
In the state of NY people downstate are usually referring to the city when they say "New York", people upstate are usually referring to the state when they say the same thing.
So even in english it can mean either depending on the context of the conversation when it isnt specified
DL isn't very good at abbreviations even in courses graduated from beta.