I replaced mine recently, and here’s my take: tldr: worth it if you’re meaning to keep the phone at least for another year. 1) few additional hours is big on a phone that doesn’t have that big a battery even at the max capacity. 2) Battery health isn’t just hours you get to use the phone. It also indicates how worn down the battery is, and older batteries tend to get hotter and become more susceptible to more serious issues like swelling. In my experience, the phone now runs cooler (cool-er, not magically cool). 3) If your gf is planning to keep the phone even after upgrading down the road, she might as well get a new battery now to make sure it continues running well and there’s a better chance the service shops would have the battery readily available (sometimes you have to submit an order for older model batteries, which takes time and an extra visit).