What is Herbert's fiance's father's ailment?Herbert's fiance is Clara.
In Charles Dickens's Great Expectations, Pip meets Herbert's fiancee, Clara in Chapter XLVI when he goes to Mill Pond Bank to locate the boarding house where Provis is staying. There he is greeted by Herbert and Clara, whose father lives upstairs because he rendered an invalid with gout. Herbert tells Pip,
"I am afraid he's a sad old rascal....Don't you smell rum? He's always at it....and you may suppose how mild it makes his gout."
Clara's father has gout, a painful form of arthritis, in several parts of his body. So, he "keeps his provisions" upstairs with him; to relieve the tremendous pain, he drinks heavily. Because of the pain, he is exceedingly cranky, as Pip remarks that he becomes aware of an alarming growling overhead. Nevertheless, the sweet-tempered Clara waits upon him solicitiously.
The example of Clara's attendance upon Mr. Barley parallels the temperate nature of Joe as he has lived with his termagent, Mrs. Joe. Both Clara and Joe illustrate the true meaning of love as does Wemmick who cares lovingly for his father and in contrast to the distorted idea of Miss Havisham that love is "blind devotion" to the injury of oneself.
I believe that you can find the answer to this question in Chapter 46 of the book. It seems that Clara's father (Mr. Barley) has two "ailments." One is that he has gout and the other is that he is alcoholic.
We can see this in a couple of places. One is where we are told that all he can talk about is "Gout, Rum, and Purser's stores." A bit later on, we see what he is having for dinner and we are told that it is good for people with gout. Finally, we see Mr. Barley drinking a lot. Clara has to go up to his room and get him his "grog." He then seems to be singing drunkenly.