The question asks if we can use either the Zero-article or indefinite article to reference an 'already-mentioned noun'. An example of each was supplied by means of a sentence pair. This is good, because questions about article usage are best asked with regards to a particular context. Thus, this answer will attempt to limit itself to the provided context.
I was walking with some friends. Friends were very funny.
It is not a natural use of Friends to refer back to some friends. To naturally do this, we would normally use one of three determiners that indicate definiteness:
a) the definite article, the, which you did not want to use
b) the demonstrative pronoun, these
c) a possessive pronoun, such as my or our
Despite the lack of any of these three types of determiners, we would probably interpret Friends to refer to some friends, but we would do so only because that is the most likely meaning of this unnatural usage.
We use the Zero-article before a plural count noun in the following:
1 in a generic noun phrase
Ants have lived in Africa for centuries.
2 to indicate an indefinite amount
Ants were crawling all over our picnic basket.
I woke up and all I could see were trees.
In your sentence pair, you mention some friends. This refers to specific friends, but it does so in an indefinite manner (thus this usage of some functions as a plural indefinite article). Since you have referred to specific friends in your first sentence, in normal daily discourse it would be rare for Friends in the second sentence to have a generic reference. Therefore your listener or reader would most likely interpret Friends to refer back to some friends. But they would do this by default. And your listener would also characterize this as nonnatural and ungrammatical.
We made a stop. After making a stop, we moved on.
It is possible for the second a stop to refer back to a stop, but this is most likely due to the similarity of construction ('made a stop' and 'after making a stop'). We use every possible clue to interpret meaning, and here the similarity in construction allows a listener to infer that the speaker intends the second a stop to refer to the first a stop. Still, this is only an inference.
In a dissimilar construction, the interpretation is even less likely. For instance, in
We saw a blackbird. A blackbird flew away.
It is much less clear (if clear at all) that the second a blackbird refers to the first a blackbird.
In both cases, the speaker would normally use one of three determiners of definiteness to make it explicit that he is talking about the same stop, or blackbird:
We made a stop. After making the/this/our stop, we moved on.
We saw a blackbird. The/this/our blackbird flew away.
The following example is intended to show that my answer does not begin to cover every single possible use of articles.
There are times when, with continued use of the indefinite article, the same noun can be referenced. Consider:
I tell my family that I am going to see a (certain specific) Porsche today. I go to the car dealership and say 'There's a Porsche here that I want to see.' During the test drive, I say to myself 'I came to see a Porsche and a Porsche I have seen.' I thank the dealer for showing me a Porsche. I then go home and tell my family, "Well, I did it, I went and not only saw a Porsche but I drove a Porsche.'
That is 7 straight uses of a Porsche that refer to the same Porsche every time. These involve the use of a Porsche to mean a specific Porsche, a certain Porsche, and/or the assumption by the speaker that his listener knows which Porsche he is referring to when he says a Porsche.
Indeed, it is what the speaker assumes about his listener, and what he might want to reveal or hide from his listener, that determines article usage. This is true, whether or not the speaker's assumption is true. In addition, speakers have idiosyncratic uses. As such, these uses can override the norms that grammar describes. Furthermore, language allows for creativity. A speaker may 'toy' with his listener with his use of articles.