The problem does not lie in "giving negative feedback", the problem lies in your friend's feelings about their self-worth and self-esteem, which are apparently impaired, given their inability to respond to a relatively common interpersonal situation. The recording and "proof" are likely to be useless, or to be used against your friend as evidence of some sort of personal grudge.
Chances are, "that person" will be your friend's supervisor pretty soon - so perhaps your friend should buy them some nice Christmas gifts and practice sucking up.