Thank you for your query.
As we are unable to see a photograph of the patch described by you it is not possible to give you a definite answer on the reason for the patch. Some areas are usually hyper-pigmented and nothing needs to be done for it. As the patch does not appear in summer due to tanning
then all the more reason for you not to worry. In the winter months they can be masked by facial make-up.
I hope I have responded to your query to your satisfaction. Feel free to revert back in case you require more clarification or want to share more information.
Wishing your daughter all the best.