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.
Washing the feet and ankles is one of the obligatory parts of wudoo’. 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 wudoo’. He washed his face and did wudoo’ 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 wudoo’.
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 wudoo’ 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”
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‘ Fataawa 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)
What must be done is to include the ankles and other parts that are to be washed in wudoo’; if the individual omits any part of that, he must repeat the wudoo’ 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 wudoo’ 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 wudoo’, and if he prayed before that, he must repeat the wudoo’ and the prayer. End quote.
Fataawa 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 wudoo’.
For more information, please see the answer to question no. 11497
And Allah knows best.