Validating the fact that she's in pain can go a long way. It's hard to be a 13-year-old girl.

Teenagers are young humans that are hitting growth spurts, getting blasted with hormones and are known for being weird, awkward, and mean.

Girls get watched like hawks and anything but pleasantries gets blamed on their periods, while young human boys, well they just get to be teenagers learning how to fit into society.

Maybe read into internalized misogyny. We don't need to hate ourselves and blame our periods when we don't want to put up with bullshit. Teenagers need to learn how to express themselves without bottling it up.

Also it's not your job, you can let your parents handle it.

Here's a good take in sitcom form by good ol' Clair Huxtable if you need it.

https://youtu.be/VwGNoPudceQ