The argument goes that Norathar, having been forced out of the House of Dragon, would have sought to learn a new trade in the House of Jhereg. Having determined that her skills and temperment suited the role of a Jhereg assassin, she would have sought out the best known assassin, i.e. Mario, to perfect her skills in this vocation.
At the time of Kragar's expulsion from the House of Dragon, Norathar was herself a disgraced Dragonlord. It could be that she sympathized with Kragar's situation, and introduced him to Mario, where he then became Mario's contact for setting up assassination jobs.
While there is no hard evidence for any of this (making it pure speculation of the basest kind), it does fit in with the characters and tendancies of all involved.