Thanks so much for the replies guys, do u think she should just get a desktop then? She's got her heart set on a laptop, so u think she would have a better chance of running it on the MacBook pro?
As others have stated, Sims 3 will run fine on a MBA. I haven't played it in a while so there might be a point in which you'll notice a slowdown, but that shouldn't take precedence over buying a laptop when she wants the portability.
The argument about "hardcore" gaming is somewhat academic, and really has no relation to the situation you originally asked about. An MBA will be fine.