Do you have any other bans set by IP addresses? It sounds like she might be getting caught in an IP block. For instance, if you banned someone by a number that happened to be AOL - those numbers change constantly and are assigned out to its members randomly so the end result is that the person you originally meant to be IP banned won't be....and another user who just happened to have been assigned that number another day could be banned.
That may not be the problem at all, but it's the first thing that comes to my mind and would be what I would check first. See if her IP comes close to any others you may have banned. If that's not it though, please do post again here.