Is there a particular mourning period (i.e. number of days) when a person’s father has passed away? Does the person have to observe the forty days of mourning for him? Also, my mother is 81 years old, does she have to observe Iddah?
There is no specified length of time to mourn for a deceased father. However, it is makrooh (reprehensible) to mourn over any dead person for more than three days.
Your mother must observe Iddatul-Wafat, which is four months and ten days, and during that period she must avoid any adornment such as wearing bright colors, make up, etc.