Well, you have mentioned that Apple did most of the work and
Rain did little.
Put yourself in her shoes: If you are the one who did most of the
work, and then, suddenly, the person who did the least work accused
you of not doing the work in front of the whole class, how would
you feel? She treated you as her best fren, so, in her mind, she
thought that you are the one who understands her the best, and
should be standing up for her as she was wrongly accused. Well, in
a way, i have to admit that she is immature by running out of the
classroom without listening to Rain's explanation.
In your shoes: you are the type of person who wants to avoid
confrontation. Thus, you tried not to take sides. But the more you
do that, the more Apple felt that you are not treating her as a
best fren.
Apparently, Apple is still hurt by the whole incident. That
explains her actions.
