For a 100g tank, I'd go with a cannister filter. They are easy to maintain, very quiet, and better for filtering larger tanks. I'd go with a Rena XP3. Its rated for a 175g tank, so it should do the job on a tank that large. If she went with HOB
filters, she'd probably need to get 2. Cannisters are a little more expensive, but they come with almost all the media you will need. I suggest buying some bio-stars/balls or ceramic rings to build the bacteria on. They don't need replacing.