Forty days is an extremely long time for a water fast. It's amazing that your mother was able to do it! I'd think by even day three or so, you'd be weakened and in danger of flushing all the electrolytes out of your system. There are quite a few members here who have fasted over a long period of time (though I am not sure they went forty days) and I am interested to hear what they have to say.
May the words of my mouth and the meditation of my heart
be pleasing in your sight,
O LORD, my Rock and my Redeemer.