Answer:

A person has to fear Allah as much as one can according to his/her ability in trying to perform obligatory acts. As it has been mentioned in the sound tradition of our beloved Prophet of Allah (peace be upon him);

"Whatsoever I have declared prohibited then stay away from it, and whatsoever I have obligated upon you then try to do it as much as you can" [agreed upon]

Thus, a Muslim should perform acts of ibadah according to his capability. As for the question itself now, there was a similar incident which took place in the time of the Prophet of Allah (peace be upon him);

Narrated Jabir the saying of the propget to a sick person who prayed on a cushion: the prophet threw it away and said, "pray on the ground, if you are able to do so; otherwise, pray by gesturing signs and make your prostration lower than your bowing" [Al-Baihaqi reported it through a strong chain of narrators]

There is another tradition of the prophet to another companion in which he directed him;

Narrated Imran bin Husain; Allahs Messenger said, "pray standing and if you are unable, pray sitting and if you cannot, pray lying on your side, otherwise pray by signs" [Reported by Bukhari]

So, in the light of these instructions we can suggest that your mother should perform those actinos which she can such as standing for takbeera tul ahraam and for rkuoo she can sit down and tilt a little towards the ground making it lesser than her tilting for prostration. She shouldn't use any table or anything else to perform sajada upon just as was indicated by the prophet to the person who used a pillow for the use of sajdah.