The problem is that, unless you give your friend your disc to play, she will have to... get around it
somehow (I don't think it's allowed to explain how here). If that's so, she won't be able to update the game and, if she tries to do it, the launcher will detect that she is playing without the disc and the game won't work properly anymore.
I did it once with Ambitions because my disc was scratched I didn't know about Origin yet, and then forgot to turn the automatic updates off. It turns out that the game not only stopped working, but also, after a complete uninstall, the game wouldn't reinstall because it said it wasn't an original copy. My discs are all original (again, I only did it because the disc was scratched) so my guess is that the game somehow alters the registry of your computer when it detects piracy. I had to follow some very detailed and scary instructions to go into my laptop's registry, delete everything sim-related there and then install the game again. I'm no tech savvy whatsoever so I really don't understand what happened, I just followed the instructions and hoped not to destroy the laptop.
But what I mean with this is that your friend would have to be careful, or play with your CD, or, as someone else said, buy an expansion pack so that she has a CD to play with. The Sims 3 is an extremely complex game so it allways has new bugs and such, so the updates are really necessary and I wouldn't recommend playing without them. I'm not saying you shouldn't do this, but I thought it was important to mention it so you know the risks
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