I used to do scanlation! I was pretty annoyed when people would compare their sorry ass translations with mine since they would get everything wrong.
What I would do is have the raw (japanese page), their translation, and mine side by side. Then look through it and point out every mistake they made. (I had a lot of time ok?)
How do you know if they actually can do translations? What she should do is point out their mistakes, explain WHY they are wrong. That the most joy I had… pointing out their flaws. It was really self-satisfactory. Even if the other side was adamant that it was all about ~interpretation~
I would ask her to explain HOW they are wrong. I just feel mad for the people who have no other means when it comes to reading manga. Some people’s translations can be god awful.
Good luck! Let us know what happens.
God knows, I love knowing other people who like to correct other people’s translations.