You may want to show her this interesting article in the Watchtower...
Watchtower February 1881 p.188
If we were following a man undoubtedly it would be different
with us; undoubtedly one human idea would contradict another
and that which was light one or two or six years ago would be
regarded as darkness now: But with God there is no
variableness, neither shadow of turning, and so it is with truth;
any knowledge or light coming from God must be like its
author. A new view of truth never can contradict a former truth.
"New light" never extinguishes older "light," but adds to it. If
you were lighting up a building containing seven gas jets you
would not extinguish one every time you lighted another, but
would add one light to another and they would be in harmony
and thus give increase of light: So is it with the light of truth; the
true increase is by adding to, not by substituting one for another.