I wouldn't change it ,but I'd ask her what style she wanted to achieve and then show her some examples of the use of for taken from a concordancer
. Here's a few I prepared earlier ...
1. You would not know it to look at him, for he has survived virtually unscathed in a testing
2. ...however, the story moved into a new phase, for he was then committed to the Crown Court and
3. ...Gooch in a highly embarrassing position, for he often seeks out Boycott for technical advice,
4. He was swearing. I feared for my safety for he was trying to tear my office apart.
5. He will print them, out of doubt; for he cares not what he puts into the press.
I'd show her how there are clues even in these short texts that the style is formal or even slightly archaic - the negation in (5); the choice of vocabulary - seeks out
rather than looks for
, or feared for my safety
rather than was scared
; the use of full forms rather than contractions. The choice of for
rather than because
adds to this sense of formality
And if she had said that she wanted a more neutral, or informal, style, then I'd suggest using because
instead. If she said she wanted a formal style - then you might discuss whether that's approriate to the type of text she was writing.
Hope that helps...