Yes, of course you could just do one layer and mask. But if you do it that way, you have to mask every.single.strand.of.hair.on.her.head. And that would drive anyone to insanity. This way, you only have to mask the hairs you don't want.
Have you read this?
You actually did the right thing here. If you'd remembered to bring a comb, this would have been dead easy.