The girl didn't make a mistake. A mistake is spelling a word wrong or using improper English like you have done. She killed someone. Doesn't matter if they were fighting or if the other girl started it. She ended up stabbed and dead - that makes it murder!
It is good to support your friend but get your head out of the clouds. She made the choice and now she has to face the punishment. I am glad she has asked God for forgiveness, but that doesn't mean she shouldn't serve time for what she has done. God still expects us to stand up and admit what we did and then suffer the consequences of our choices.
Be a good friend, support her and when she goes off to jail if she is convicted, write her and visit her. Let her know that you support her, but for goodness sake, if you haven't finished school do so. Make something of yourself and be responsible. It could happen to you too!
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