In that case, Louis should have no dukedom either and be known simply as HRH Prince Louis of Wales when Charles is king and William is PoW, and later as HRH The Prince Louis when William is king.
I can't predict the future, but I find that unlikely. I believe either Charles or William (whoever is the King at the time) will make Louis a royal duke when he gets married.
In an age of sex blind succession I'd be surprised if only Louis was granted a dukedom & not his sister.
More likely that neither would be I think. I suspect they will both be offered the option of being entirely private citizens with the ability to abjure their styles & titles if they want.