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.