I'm guessing that those are steel rims on the bike now. If so, Sun CR-18 alloy 650A rims weigh (slightly) less than the corresponding 700c rims. Panaracer Col de la Vie (available in 650A as well as 650B) weigh about 390 g, which is reasonably light. Going in this direction would be easier, not require a fork swap. In addition, dropping down to 23's might be uncomfortable for her. I wouldn't be surprised if the bottom bracket/crank and the cluster offered greater opportunities for weight reduction.