My mother underwent a knee surgery; thus, she finds it hard to perform ablution and prayers. Can she combine prayers?

Answer

Praise Be to Allah and Peace Be Upon His Messenger.
Referring to your question, some Muslim scholars allow Muslims who are sick and find it difficult to perform each prayer at its due time to combine prayers. 
Shaikh Muhammad Ahmad Hussain, the General Mufti (Muslim Legal Scholar), in the Palestinian Legal Verdict Council says, originally, a Muslim should perform each prayer at its due time. Allah says: “The prayer is enjoined on the believers at fixed hours.” An-Nisa’: 103. 
Abdullah Ibn Mas’ud narrated that he asked the Prophet Muhammad (PBUH); “Which action is dearest to Allah? And the prophet replied: “Prayer at its proper time”. Related by Muslim.  
However, Islam allows Muslims to combine each two prayers due to certain circumstances. Scholars have no agreement on what these circumstances are. Hanafi School scholars say that combining each two prayers is not allowed but for pilgrims on the Day of Arafat on the Mount of Arafa and on Muzdalefa. during performing Hajj. (Hashiyat Ibn Abdeen: 1/555)
The majority of scholars allow Muslims to combine each two prayers in other situations once there are considered excuses such as rain, fear, or illness. (Hashiyat Ad-Dusuki: 1/396) (Rawdat At-Talibin: 2/70) (Al-Mughni: 2/205). 
The Hanbali School scholars have widened the range of excuses that allow Muslims to combine prayers. They say that excuses which allow Muslims to combine prayers are similar to excuses which allow them to leave the Friday Prayer such as being fearful for himself, money or property and others. Ashhab, who belongs to the Malki School, and Ibn Al-Munther, who belongs to the Shafi’i School, Ibn Sirin and Ibn Shabrama think that it is allowed to combine prayers when it is necessary as long as this does not become a habit and routine. (Al-Mughni: 2/205). Ibn Abbas (RAA) narrated: “The messenger of Allah (PBUH) prayed Dhuhr and Asr together, and Maghrib and Isha’ together, and not out of fear nor because of travelling.” Related by Muslim. 
Then, Ibn Abbas was asked, “why did the prophet intend to do that?” He answered that the prophet did not want to put his Ummah to unnecessary hardship. 
Thus, if your mother finds it hard to perform ablution and prayers at different times, each prayer at its due time, she can combine each two prayers.