Rounding from day one builds terrible habits that are hard to fix later. Just because it's common doesn't mean it's safe or right. That little bit of rounding puts unfair stress on the spine and can lead to real injury over time. It's better to lower the weight or even do a simpler exercise to keep a flat back. Why risk back pain just to follow what most people do wrong?
Honestly, a little back rounding on heavy pulls is pretty normal, even for pros. The "perfect flat back" thing is kinda overblown for beginners. If she's just starting, the main goal is to keep her safe and having fun. Maybe just focus on cues like "chest up" or have her practice with a light kettlebell to get the feel. Fixating on a tiny curve might make her nervous and ruin the whole point of getting her moving.