@bockerino because she isnt lady fortnum, had to check in an agregator site since they have up to chapter 90 and here the translators changed her name.
Wouldn't be surprised if its the prince again that crashes into the place at the most inopportune moment and 'offers' to be the dancing partner, of which then Alphones sees by chance and gets a huge pang of jealousy.
I must confess I am guilty of reading ahead with bad scanlations on another website. It always gives me peace of mind to read these high quality versions though because the story is always so much clearer when i do.
I think it's just gonna be Alphonse who volunteers to be her partner because why would the prince even be there. Alphonse most likely knows her schedule and is very attentive of her. Additionally, Chloe is not aware of any guests, and I think Alphonse would take notice if some were to show up uninvited.