I think Boris Johnson is a most unpleasant man and not fit to be prime minister. There are enough verifiable accounts of his boorish behaviour without the need to rake up stories from 20 or so years ago. Whether this woman's account is true or not, only she will know, but her decision to talk about it now was guaranteed to draw criticism, and one wonders what the motivation for this superfluous revelation was.
As for the story about Corbyn:
1 - who is the courageous "witness"?
2 - motivation, i.e. "showing off", is ascribed to this incident but that is just an assumption and speaks more of the "witness" than Corbyn.
3 - The quote from the witness about this alleged event was:
"It was [the] late ’70s, it was still a point of interest, a white man with a black woman, so he was slightly showing off: ‘I’ve got a new girlfriend, and she’s black’.”
Whoever this so-called witness to this alleged incident is, this remark demonstrates his racism. Incidentally, inter-race relationships were not uncommon in the 70's in the areas of south-east of England where I lived - apart from myself, I knew many people in such relationships. In London, it was even more commonplace. Again, the comment tells us volumes about the person who made it.