Like the others said, there isn't a proper width. It all depends on the rider and their riding style. So if/when you are cutting, do less than you think. Even small cuts off of each end will make more of a difference than you'd think. So make small cuts and creep up on it like scrublover suggested. But like unsuspended said, let her ride it first and see if she likes it as is. With a good tube cutter, it's quick and simple to cut the bars while on the bike if she finds she does want it narrower.