All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If the discharge that she experiences is semen, then her fast becomes null and she has to make up that day in later days. If the discharge is Madhi (pre-seminal fluid), then the same ruling applies to it according to the Maaliki and Hanbali Schools of jurisprudence.
If the wetness is caused only by the thought in sexual matters, then according to the most correct opinion, it does not have any effect on the validity of the fast especially if one does not indulge oneself on such thoughts for a long time. This ruling is based on the Hadeeth of the Prophet that he said: “Allah has forgiven my nation for the whispers that they have except if they speak about it or act according to it.” [Al-Bukhari and Muslim]
As for performing Ghusl for this discharge if it is only pre-seminal discharge, then one needs only to wash it off and make ablution. But if there is ejaculation (i.e. discharge of sperm), then performing Ghusl becomes an obligation.
Allah knows best.