70% chocolate from 53% and unsweetened
She could mix them, and it would be about 8:2 semi to unsweetened.

  8 part semi - 56 units cacao
  2 part unsw - 20 units cacao
  ----------------------------------
  10 parts mix - 76 units cacao, or about 75%


Still, most cake recipes are very forgiving, and the unsweetened chocolate will have a different flavor and possibly smoothness than the semi-sweet, so it may not be worth it.