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First thing is first, set your mother up with a different account. If she never needs to install applications then set it as a standard account with otherwise full/normal access to everything else. If needed, make it a Simple Finder if she only uses, say the internet and mail, with only those specific apps available.

Secondly, yell, "GOSH MOM, JEEZ WHAT THE HELL!", and make her feel bad. Make sure you mention the loss of school/work documents and and/or pictures of friends/family. If you make her feel guilty enough, start suggesting, although not directly, that you get another computer for yourself to use to ensure that this never happens again. "IF I HAD MY OWN COMPUTER THIS WOULD HAVE NEVER HAPPENED! GOOOOOOOOSSSSSSHHHHHHH JEEEEEEEEEEEZZZZZZ! I CAN'T BELIEVE IT! I STILL LOVE YOU MOM, BUT C'MON!? WHAT WERE YOU THINKING!"

That's all I can suggest :p
 
Secondly, yell, "GOSH MOM, JEEZ WHAT THE HELL!", and make her feel bad. Make sure you mention the loss of school/work documents and and/or pictures of friends/family. If you make her feel guilty enough, start suggesting, although not directly, that you get another computer for yourself to use to ensure that this never happens again. "IF I HAD MY OWN COMPUTER THIS WOULD HAVE NEVER HAPPENED! GOOOOOOOOSSSSSSHHHHHHH JEEEEEEEEEEEZZZZZZ! I CAN'T BELIEVE IT! I STILL LOVE YOU MOM, BUT C'MON!? WHAT WERE YOU THINKING!"

That had crossed my mind as well. :D

If you did have any important files on there, you could purchase a data recovery app and it may be able to get some stuff back. If you want to do this, do it now and don't use the computer until you've recovered everything you can. Your data is still there, but the OS considers it to be free space, so it'll overwrite it whenever it likes.

And for the sake of God back up in future! There's no accounting for incompetent mothers.
 
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