They are not going to be damaged for life, they are not going to be scarred, they will mostly stop doing it to others
There is a possibility that Crust is being taken out of context. I don't subscribe to this method of parenting with my dc (or others!), but I imagine Crust is talking about a one-off, whereas some people seem to be imagining her doing it repeatedly to her kids all the time.
My own experience is that it was done to me as a small child - I used to bite my brother, and although I was told not to I kept doing it anyway. Eventually my mum bit me to show me what it was like, and (again, I wouldn't choose this way myself) I never bit my brother again. Quite possibly because I didn't fancy having it done to me, rather than because I didn't want to hurt him, but the point is I was only bitten once and never did it again. I also don't have any particular feelings about it, don't feel scarred; it is a quite indifferent part of my childhood.
I wouldn't take this approach myself because I wouldn't feel morally comfortable with it, but I suspect that what Crust is referring to is similar to my own experience. It's like the difference between a tap and a beating. I wouldn't do either, but I can recognise that a tap in context isn't necessarily a cause for such concern as shown here.
Anyway, in regard to the OP, it's pretty obvious that your friend was wrong to do what she did!