The tyre won't work quite how the manufacturer intended. You could run off the edge of the tread, or find that the tyre steers like a triangular Dunlop. You could get funny handling due to too much or too little sidewall stiffness. You'll probably want to avoid extreme pressure setting at either end of the scale. You're more likely to find that you can't find a pressure which gives good ride and good handling. The tyre might be more likely to come off the rim when punctured. That sort of stuff.
Unless I come up with a disc-brake front wheel for £0 she's going to have to stay in the green, which is handy because I bought a pair of 35mm Hypers for her bike anyway but thought I might be able to get her some fast 28s.
28s on 25mm rims is green on my chart. Since the Hypers already roll pretty well, the big gain from getting an actual road race tyre is likely to be aeros and weight.
The tyre won't work quite how the manufacturer intended. You could run off the edge of the tread, or find that the tyre steers like a triangular Dunlop. You could get funny handling due to too much or too little sidewall stiffness. You'll probably want to avoid extreme pressure setting at either end of the scale. You're more likely to find that you can't find a pressure which gives good ride and good handling. The tyre might be more likely to come off the rim when punctured. That sort of stuff.