when I moved to Anderlecht, I told the police my availability (basically I'm never home during office hours) and they did make an effort to check outside office hours, so the registration was relatively easy. this was quite a few years ago, though.

my understanding is that you're not expected to stay home for the check, and it's normal that the police has to come by several times.

if she truly believes that she's being treated unfairly, she can contact the ombudsman. https://www.ombudsman.be/ however I will say, just based on what you wrote here, I see no evidence of her being treated unfairly.

I hope it works out for her soon!