People who buy pets for other people are absolute scum. Hey, you can spend half your life caring for this thing you don't exactly want or kill it, your choice.
I don't know if there is any good way to break it to your GF that this was absolutely not okay and a highly stupid thing to do but try your best.
This is essentially how my parents ended up with a turtle. One of my sister's college roomates bought 4 turtles for another roommate for her birthday. Well, the illegal Chinese turtles were not supposed to be sold at a young age and 2 died fairly quickly. The other one eventually died when the roomate who received them got bored and basically stopped caring for them (feeding and cleaning tanks is something that needs to be done but it takes time and effort). Note that a 3rd roommate also got turtles from the same place and never got a sun-lamp so their shells turned to goo and they all died. My sister felt bad and took the remaining one but even she couldn't find the time to care for it so she left it with my parents. Luckily my dad took a liking to it and they've been caring for it ever since and have bought a couple hundred dollars in aquarium stuff to do so. This was 6 years ago. Also note turtles live a long time, like 30+ years, meaning it's likely they may not even be able to care for it at some point in the future. Also, the type of turtle, red-eared sliders, are a super invasive species that have proliferated mainly because stupid people buy them, can't take care of them and release them causing huge amounts of destruction to local ecosystems.