I'm guessing your friend probably downloaded osu! "stable" (the client you're probably used to playing with), which does not support macOS.
She has two options. Either:
1. Download the "osu!(lazer)" option from the
download page and install that. If you're not familiar with lazer, you can learn about it
here.
2. Download this
Wineskin, which will allow her to play osu! stable. This can be a bit finnicky, but it should still work.