All of this entirely depends on why she does it and how severe it is.
Most of the time it's a sign of extreme mental unwellness. If she's doing it to cope with emotions she can't handle, you're going to have to tackle that first.
People are retarded, so they always just see the cutting first and don't think of what's actually important.
Your question isn't even asking the right thing.
Who cares about getting her to stop?
Will it be any better just because she doesn't cut?
By all means, if it's severe, that should probably be the priority if you care about her.