Tires I like are the Rene Herse formerly Compass tires. These are supple tires and you immediately feel the difference. If you purchase the Chinook Pass extralights they are only 229 grams. Use tubes with these, they are not tubeless design.
At her light weight, she won't need a lot of pressure in those tires. You could read
this and it will give you something to guide you.
As other have said, lighter wheels will is always the best bang for your buck but can also be the priciest.
Depending on how much she rides, a fun thing is a bike computer like Wahoo or Garmin. It won't make the bike any lighter but it is fun keeping track of your metrics.
Congratulations to you both on your new bike purchases.