彼女: You'd better eat that pizza until tomorrow.
彼女: We bought it today, and if you don't eat it tomorrow it will go off. So you'd better eat it until tomorrow.
私：ちがうちがう。You mean "You have until tomorrow to eat it" or "You should eat it tomorrow". What you said was 明日まで食べてほうがいい。
I had to explain that the English "eat until tomorrow" meant "start eating now and don't stop eating until tomorrow", and she had to explain that the Japanese 明日まで食べる meant "you have until tomorrow to start eating it".
This use of まだ is quite different to the English "until", can we have a lesson on it?
As you know by this point in your Japanese studies, languages often do not map one-to-one with each other, and in unrelated languages like English and Japanese, they seldom do. (Even words that are adopted from English into Japanese often take on quite different meanings.) In English we have at least two words that map to 「まで」: "until" and "by". Your 彼女 should have used "by" for that particular sentence construction. But in this case, it might be English which is unusual. even German, a close relative of English, uses only one word, "bis", to express both meanings. Perhaps this is a job for Englishpod101.
Are you sure she didn't just assume you meant までに, because that unambiguously means 'by'. I've never seen まで used on its own with that meaning, and none of my dictionaries mention it. Seems like a prime case of lost in translation to me, but maybe you'd want to clarify that with her. Could be some sort of casual thing, but I can only speculate.