I am really interested in what your friend meant by "who says plasticity is good?". Plasticity seems to me to be seen as a quality in psychology, no ifs no buts.
Some interesting points raised by other comments to this post. My best guess is that he meant that you could have a traumatic experience afterwards that leaves you worse than before. Maybe (just spitballing) whatever mental defences you have against trauma are enhanced by not being too plastic.