I want to ask about salat for someone who is sick.A Muslim brother said that you cann't use a chair to sit down you must sit down on the floor. but i can do my salat standing. My problem is I can't make djoulouss because I have problem on my feet. Please tell me even I can sit down on the floor, can I use the chair to pray?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

An incapable person is permitted to perform the obligatory prayers while sitting on the ground or in a chair. Also, one can perform optional prayers while sitting even if he is not incapable.
al-Nawawi (May Allah's Mercy be upon him) said in his book 'al-Majmu': 'It is sufficient for a person to perform the obligatory prayers while sitting due to incapability or the optional prayers even he is capable, and there is no specified state for sitting.'
If one's sitting on the ground helps him prostrate while sitting on the chair does not help him do that, then he has to sit on the ground. If one cannot prostrate on the ground in both above-mentioned states of sitting, it is better for him to pray on the ground due to being nearer to the original place of prostration while making an indication of prostration. The Prophet (Sallallahu Alaihi wa Sallam) said: "Do what you can of the orders I give you." [ Bukhari and Muslim ]
Then, he has to perform the prayer while sitting on the chair, if this helps him to sit erect and bow-down and then, sit on the chair to make an indication of prostration.
Allah knows best.