She's small, which will certainly restrict the boards she has to choose from.
You may be tempted to buy a children's board - don't do it. They are total crap and not built for adults.
At 100 pounds, you she would probably be most comfortable on anything between 138 and 144, with a 140 or 142 being ideal.
Honestly, her shoe size may have more of an impact on what she can ride than her weight. I wear a women's size 4 boot and every board on the market is at least one inch too wide for me. It robs me of edge control to the point where I can't ride anything stiffer than a park board.
Anyway, check out the Roxy Ollie Pop, the Roxy Eminence, the NS Pandora, the Capita Space Metal Fantasy, and the Gnu B-street. I have ridden - and loved - the NS, Capita, and Gnu boards listed here, but the NS Pandora is by far my favorite. It has everything a small girl could ask for: great edge hold, soft waist, and superior dampening (compared to the Roxy and Gnu lines).