How is it that your sister discovered this? The edge of the damage is quite fresh and clean so it was discovered right after happening. Did she hear it happen and investigate? If so, maybe she needs to recall the road on which she was riding. Rough road? Construction? Debris on road?
Since there is no evidence of external impact this should be a warranty repair but as it is damage, your sister should expect a battle. No dealership will want to go to bat for them with GM. If it turns out the damage is attributed to a missing bolt(s) on the flex plate they won't fight but there is always the possibility that there was a foreign object inside the bell housing (say an errantly dropped bolt at assembly?) that was never noticed (or reported). The proof will be in evidence of impact and / or damage from said internal foreign item.
In all cases, I wish her good luck. Please keep us posted.