TBH, If I don't see this commoner turn out to be a spy or something, I'm dropping this.
You get invited to such a event, the choosing of the prince's fiance.
So the royal family is okay with commoners marrying in? Otherwise, why invite a commoner that they never intended to marry in?
The more likely explanation is that she would be a spy employed by the queen to find out who the bad cookies are in the group of girls chosen, and aid in finding the future queen.
This would add more to both the "commoner" and the queen's involvement/role in this story, whilst also explaining why she insisted on her own punishment to test Teresse.
Ofc, this is just my theory.
I just think this story will be so bland and generic if she just so happen to be only a "friend". Also adding a minor plot hole of why the hell they invited a commoner into the event only made for possible prince fiance candidates.