"I would like to keep her bike as light as possible so that she can stay with me on rides"
I do not think knocking every last gram off the bike will change much. I would also suggest inspecting the tires for glass every 5 mi would be a good idea. You will not find much until the tires get older and less subtle.
Slime goo that you place inside the tire might also help.
2000 Montague CX, I do not recommend it, but still ride it.
Strida 3, I recommend it for rides < 10mi wo steep hills.
2006 Rowbike 720 Sport, I recommend it as an exercise bike.
1996 Birdy, Recommend.
Wieleder CARiBIKE (folding), decent frame.