Definition of the ankle and the ruling on including it when washing the foot in wudu
Question

I was travelling in the car with my father to go to Jumu‘ah prayer, and I saw part of the area above my father’s ankle bone, the colour of which was lighter than the colour of the foot and ankle bone, and it was as if it was not washed, but I did not say anything to my father.
Is the ankle included in the rough area where no hair grows above the ankle bone, or not? If it is included, what should I have done?
Praise be to Allah.The ankles are the two prominent bones at the bottom of the leg, on the two sides of the foot. Each foot has two ankle bones. 
It says in al-Mawsoo‘ah al-Fiqhiyyah (34/259): 
The word ka‘b (ankle) linguistically refers to the node between two segments of a stem. The ka‘b of a man refers to the prominent bone on the two sides of the foot. Al-Azhari said: The two ka‘bs are the prominent points on the right and left of the foot, where the leg meets the foot. 
Ibn al-A‘raabi and a number of others said: The ka‘b is the joint between the leg and the foot. The plural forms are ku‘oob, ak‘ub and ki‘aab. Al-Asma‘i rejected the popular view that the ka‘b is the back of the foot (the heel). 
According to the majority of fuqaha’, the ka‘b is the bone that protrudes where the leg and the foot meet. 
Ash-Shaafa‘i (may Allah have mercy on him) said: I do not know of anyone who disagrees with the view that the two ka‘bs are the two bones at the joint of the leg and foot. End quote. 
Secondly: 
Washing the feet and ankles is one of the obligatory parts of wudu. Allah, may He be exalted, says (interpretation of the meaning): “and (wash) your feet up to the ankles” [al-Maa’idah 5:6]. 
Muslim (246) narrated that Nu‘aym ibn ‘Abdullah al-Mujmir said: I saw Abu Hurayrah doing wudu. He washed his face and did wudu properly, then he washed his right arm as far as the first part of the upper arm, then his left arm as far as the first part of the upper arm. Then he wiped his head, then he washed his right foot as far as the first part of the calf, then his left foot as far as the first part of the calf. Then he said: This is how I saw the Messenger of Allah (blessings and peace of Allah be upon him) doing wudu. 
An-Nawawi (may Allah have mercy on him) said:
The words “as far as the first part of the upper arm” and “as far as the first part of the calf” mean that the washing includes these areas. End quote. 
It says in al-Mawsoo‘ah al-Fiqhiyyah (22/121): 
The majority of fuqaha’ are of the view that washing the feet with the ankles – which are the two bones that protrude where the leg and the foot meet – is one of the obligatory parts of wudu because Allah, may He be exalted, says (interpretation of the meaning):
“O you who believe! When you intend to offer As-Salat (the prayer), wash your faces and your hands (forearms) up to the elbows, rub (by passing wet hands over) your heads, and (wash) your feet up to the ankles”
[al-Maa’idah 5:6].
End quote. 
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: 
The ankles are the two prominent bones at the base of the leg; the ankles are included in the instruction to wash the feet.
End quote from Majmoo‘ Fatawa wa Rasaa’il al-‘Uthaymeen (12/484) 
Shaykh Ibn Jibreen (may Allah have mercy on him) said: 
The ankle is the bone that protrudes where the leg and foot meet. Each foot has two anklebones, one on either side. The ankle ends where the calf narrows on the leg. The end of the ankle and its veins, and of the area that must be washed, is where the calf narrows, so the foot and ankle must be washed up to that point. End quote. 
Sharh Akhsar al-Mukhtasaraat (2/4) 
Conclusion: 
What must be done is to include the ankles and other parts that are to be washed in wudu; if the individual omits any part of that, he must repeat the wudu and the prayer, so long as the time for the prayer has not yet ended. 
If the time has ended, then he does not have to do any of the past prayers, unless he was heedless, or it was pointed out to him but he refused to rectify the matter. In that case he must repeat the prayer concerning which he was heedless. 
The scholars of the Standing Committee said: 
The individual must do wudu properly, washing all the parts that are meant to be washed. If he omits any of those parts and the water does not reach them, he must make sure that the water reaches it. If a long time has elapsed and that part has dried off, then he must repeat his wudu, and  if he prayed before that, he must repeat the wudu and the prayer. End quote. 
Fatawa al-Lajnah ad-Daa’imah (4/71-72) 
So you have to tell your father about the shar‘i ruling, and tell him that he has to wash the ankles properly, and if he prayed without washing them properly, he must repeat the prayer, with a complete, valid wudu. 
For more information, please see the answer to question no. 11497 
And Allah knows best.