Sorry, just my opinion, which you did ask for.
You have no proof she blew that money instead of paid off debt, and why should she tell you every incoming and outgoing of her bank account?
If you had proof she had blatantly pissed the bonus up the wall then fair enough.

Money can strain friendships to the max, but try not to make too a big a deal out of her not telling you that she got the bonus, providing she has used most of it to pay off more debt I cant see any reason why she would have to tell you about it and you have no reason to question her over it.

If you are that concerned about her beginning to pay you back ask her if she wants to do another review of her finances (assuming its been a significant amount of time since you set up her repayment plan). See how her current situation is and set up a monthly standing order for her to start paying you back.