So you are going to spend $100 then drop $200 more on a bike that sold new for $330? If she really likes the bike, keep the upgrades to a minimum. You'll never make the bike light so don't worry too much about weight (the fork is heavy but should ride fine). I'd do the stem and seat post as you have those, tires because they can really help tune the ride to your conditions, bars because she wants them and do a full maintenance service to make sure everything else is OK. That's all assuming she really likes the bike.

I'd did similar to a low-end comfort bike because my wife likes the bike. She also has an Ultegra-equipped road bike and vintage MTB but still likes the comfort bike for grocery runs and errands. Do what makes her happy = you will be happy.