Any type of physical harm done to oneself on purpose is self-harm. Just because it doesn't look like the self-harm that most people who are unfamiliar with generalize it to be doesn't mean it's not self-harm. For example, loads of people generalize lesbian women as man-haters and gay men as ultra-feminine, but that sure as hell doesn't mean that stereotype isn't idiotic and hurtful to the LGBTQIA+ community. I have a friend who cut herself, but on her calves instead of her arms. I also have friends who have those scars on their wrists where most people assume self-harm scars would be. No matter what form it takes, it's self-harm, and your friend hurt herself on purpose. It's a problem that needs to be addressed. Talk to her about it and get her to go to counseling. Hurting yourself can become addictive.
Be patient with her and remind her how loved she is. Don't freak out on her or act as if it's not important. Do your best to be gentle and understanding with her. She's incredibly strong already, considering she had the courage to tell you two about it. Make sure she stays that strong. Encourage her and stand by her no matter what.
But NEVER act like she's "just making a big deal out of it" or "just looking for attention". Yes, it's a problem, and it's a big one, even if your friend can't see it. And of course she's looking for attention. She's hurting badly inside, to the point that she hurt herself just to try to make the pain go away, and she's terrified. Of course she wants attention. She's probably terrified, stressed out, and probably depressed, if not suicidal. She wants attention because she desperately needs help and she was brave enough to ask for it. Chances are, it took her an immense amount of bravery and a strong moment of self-love to be able to tell herself that she's worthy of being loved and cared for and tell two people who she values that she hurt herself. Give her that attention and help.