Help. Please.

Okay. I really need help with this. Please. Any solutions you have.
My friend cuts herself. She recently stopped, but two days ago she couldn't stop herself and she splurged. She has fresh cuts all over her left arm, from her elbow to her hand. Our high school band has a mandatory concert to play at. The uniform concert dresses are short sleeves. The concert is in 6 days. Does anybody at all know how she can hide her arm?

And please, don't say she needs help. She sees a therapist and has been hospitalized twice and this is not usual. She has been through so much; you honestly cannot imagine. Believe me when I say that. I'm just asking if anybody know how to hide it. Any suggestions are welcome. This really is my last resort. Thank you.