"Girl Friends" is not subtext, it never was subtext, it was not written as a subtext work. Gradually falling in love is not subtext.
That's not really what the word "subtext" means, but if you want to use it that way then there's no reason to think this series is "subtext" until it ends.
Under the real meaning of the word, Girl Friends was subtext until chapter 10.
No, it was not. "A hidden or less obvious meaning", according to Cambridge Dictionary, "the implicit or metaphorical meaning", according to Merriam-Webster. There was nothing 'hidden' or 'metaphorical' in "Girl Friends", the romance was not a 'less obvious' theme. If it was, then pray tell, what was the main theme? Seriously, why these discussions always arise when subtext is in question? Just because the characters are not fully aware of everything, or have not yet developed those feelings (love does not just pop out in a vacuum), does not mean the work is subtext. By that logic, I could say the first 20-odd minutes of "Zulu" are not a war film, because they do not feature anything connected to the actual war, since the people are not yet aware the conflict had begun. People seem to confuse how the characters deal with something (not being aware of their feelings, or only just developing them), and how the work (that is to say, the author) deals with it.
As for the series being subtext or not being decided at the end, that does indeed hold water. In these cases, I just presume, personally, that the scanlators have an insight into future chapters. Not the most reliable presumption, I will grant you.
If Girl Friends ended on chapter 8 you are telling me it wouldn't have wound up with a subtext tag on here?