I have no idea what it is because I don't follower her that closely, but it doesn't look like a birthmark, it's too big to be freckles or moles, too rough to be surgical, and too irregular to be decorative scarification. It looks like the kind of scar you get when you get a rough cut/stab or maybe a burn.
You're all wrong, it's a slash wound. Stabs, pending on the knife leaves a clean cut. A slash like that is probably from a serrated knife. My aunt is a ward matron for the past 38 years. I know the difference between a stab and slash wound.