This is super tricky. Asking to be compensated at the same rate as people of a different gender is very different from asking for a raise, so there are two separate conversations that need to be had. She could go to HR and ask for an explanation why she makes less than her male coworkers. There may be a plausible explanation (thought that is unlikely). It doesn’t matter how she found out that the dudes are making more; when HR asks how she found that out, she should say that it doesn’t matter. Once she has that explanation, if it is unsatisfactory, I would contact an employment lawyer to discuss her options. There may be an opportunity to get a settlement in the form of backpay without going to court for overt sexism, which is incredibly difficult to prove.
The conversation about asking for a raise based on her performance should be had during her annual review, at which point this issue is hopefully resolved.
I am very curious what askamanager would have to say about this.