Sorry to hear this City. What you need to do is get your mum some help. Don't blame her or try to come up with answers from her as to why she tried to do this...just try to help her get through it. I know how bad depression can be. It takes away your ability to apply rational thought. Many people trivialise depression...but it is a serious thing.
I have had friends tell me to 'just get over it'. They have no clue and no concept of how low one can sink when depression sets in. Make sure you comfort your mum and talk to her. Don't try to offer solutions - just be there to listen. But definatley get her into some counselling.