Allah (SWT)'s mercy on His creatures has no limits as He always showers His bounties on all the creatures whether they obey Him or not. His mercy is on every one and every thing. Out of His mercy, Allah (SWT) does not put burden on any one more than the capacity and the ability of that person. All the obligation in Islam are withing the limits of the ability and when a person is unable because of any reason, the obligation will be lifted from him as far and as long as he is unable to do it. For example: Fasting during the month of Ramadhan is one of most important obligatory deeds upon every Muslim, but if a person is ill and can not fast, fasting becomes not obligatory on him.
This rule applies on every situation according to the abbility of the person.