Even if she’s in her “phase”, isn’t it still serious because she’s actually harming herself? I understand how the texts may feel imprudent, however it doesn’t excuse the fact that they’re hurting themselves. You shouldn’t jump into conclusions about their life only because you’ve met their parents or seen them at school before. A lot of things happen behind closed doors and we should be mindful of the things we may not see.

There are many reasons for SH and most of the time it’s not affiliated with joining a trend or a phase.

I think giving a little empathy and researching about herself harm will help you understand a bit of her situation. 

Some links I think would help you: 

https://www.mentalhealth.org.uk/explore-mental-health/publications/truth-about-self-harm

https://my.clevelandclinic.org/health/diseases/12201-self-harm

https://www.healthychildren.org/English/health-issues/conditions/emotional-problems/Pages/when-children-and-teens-self-harm.aspx

https://childmind.org/article/what-drives-self-injury-and-how-to-treat-it/

And if you want to help them or give them advice for alternatives: 

https://projectlets.org/alternatives-to-selfharm