No sure what the cut off age is, it might be 22. Staying at a youth hostel would save her a lot of money and I don't think I was ever in a bad one. Although, she would have to share a room with a bunch of other girls - at least three or four. Even if she can't get into a youth hostel it's easy to use the tourist system to get a hotel room based on what she can afford.
I found Brussels to be a very boring city, but an easy commute to everywhere else.