Group project and one member clearly used ChatGPT for their section

Dealing with an awkward situation in a grade 9 group project, four students, one shared slideshow, four sections each written by a different member. Three sections read like actual fourteen year olds wrote them, some rough transitions, uneven vocabulary, a couple grammar mistakes here and there, totally normal for the grade. The fourth section is polished, sophisticated vocabulary, perfect transitions, reads nothing like this student’s other work all year. Pretty clearly AI generated start to finish. The other three group members haven’t said anything, not sure if they noticed or just don’t want to cause drama with a group member. My dilemma is the group grade includes a shared component alongside individual sections, so if I address the one student’s section it might tank a grade for three kids who did their own work honestly. Do I grade the sections individually and quietly flag the one section, do I talk to the whole group together, or do I pull the one student aside first before anyone else even knows there’s an issue. Don’t want to blow up group dynamics over one member’s shortcut.