Im all for the get the snake then tell your parents. Thats what I did, but I did whilei was at school, and i wouldn't have moved home for the summer if she wouldn't let me bring my two home with me.
Surprisingly enough now she is freaked out over the idea of rats in the freezer. She even freaked out at dinner cause she though pasta shells looked like rat faces.....hahahaha, it was hillarious.
My advice for you is just tell her that one of your friends got it, couldn't take care of it properly and you took it cause you would rather see it live a happy life then a neglected one. Moms love the i love animals to much to see them suffer card.