Is it considered a tribulation when a mother discontinues her prayers during times of depression, even though she makes up the missed prayers on days when she feels more energetic, and what can be done to help her?

You must advise your mother gently and kindly, reminding her of the impermissibility of abandoning prayer, as depression is not an excuse. Rather, sadness and depression often arise from being far from the remembrance of Allah, and "prayer is the greatest remembrance." Your mother must repent to Allah for abandoning prayer, unless she has lost her mind due to illness. Her marriage to someone older than her is not forbidden by Islamic law, as the criterion is not age but the husband's character, religiosity, and ability to fulfill the wife's rights. If she was forced into marriage, this is a matter of the past and does not permit her to be remiss in prayer.