First off, almost any 100mm fork can be lowered to 80mm pretty easily.
Second, going up 20mm of travel shouldn't be a big deal. As a general rule of thumb, for every 20mm of travel you add to a frame, it slacks the head angle out by 1 degree. I'd say throw the fork on the bike and see if she likes it. If she doesn't, take it to a shop and have them lower it to 80mm.
My wife's extra small has the f100 fork from fox. It actually came with the frame from Jenson. She likes the way 100mm handles. I've tried the bike too and I think the 80mm fork won't be enough travel.
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