This is a good way for him to get fired (and probably get his company sued if he mentions the off-work honesty incident when he is giving a reference for her as a representative of his company.)
The best thing he can do is pretend this never happened. He should never mention it again, never loan money to co-workers again, and if it ever comes up again then deny that it ever happened (nothing in writing, right?) He has already played favorites by getting her a raise and giving her a loan, if he does anything to seek "revenge" on her, then all of that will come out, and he will be the one in trouble for not treating his co-workers professionally. Or worse case scenario, she can make up a story about what he "expected" in return for his money, and when she didn't give it to him, he sought revenge. He made a big mistake by loaning the money, if he does anything to draw attention to this fact (including asking her for the money back), it will make matters worse. Best thing he can do is completely forget it (AND give her a good reference so she will leave and go somewhere else to work.) As long as she is there, he must be extra careful to treat her exactly the same as everyone else.