"I must warn you about her."
Translation:Jag måste varna dig för henne.
I think that's mostly just because different languages make different choices. för is often used in contexts like this, we've got förvarna which is like forewarn in English, there's this sense of 'before' in it, a bit like 'I'm warning you before you meet her, so you can be prepared'.
The same preposition is used in combinations like rädd för 'afraid of' where the thing is sort of 'causing' an emotion.
varnade om is not a natural way to put it in Swedish, it sounds like a bad translation from English to me.
This answer is in itself an odd contextual use of English, well done for bending to fit the query though. Forewarn does indeed predict future action, but then again so does 'warn' i.e. what is the use of a warning after the fact? This is why only poets and the pretentious use forewarn in modern English. In English we dont 'warn for [subject]', ever, we 'warn about [subject]', always. Hence, forevarna om or varna om makes perfect sense and (fore)varna för does not.
Perhaps all answers can be updated to read 'Swedish prepositions are irregualr. They just are. Stop fighting it. Learn them and learn to live with it!'