Many Christians get depressed. In her case as I said early --most likely it was postpartum depression--sometimes depression is chemical and sometimes it is situational. You can easily find many stories in the bible of great men of God who became depressed. Moses, Elijah, Hannah, Solomon, David, Job, just to name a few.
Depression is part and parcel of life--when it becomes extremely dark, it is like falling down a well--the person will need outside help and of course prayer to get out of it.
"For we would not, brethren, have you ignorant of our trouble which came to us in Asia, that we were pressed out of measure, above strength, insomuch that we despaired even of life."--2 Corinthians 1:8