PC and PS3 Use?

I know I have to buy a copy for each and link them on my SE account. But if my wife wants to play and I buy her a PS3 copy and register that to her own SE account, since I already own both PC and PS3 version can she also play on the PC? Or would I need a 2nd PC copy for her to do so?

She would need a PC copy if that is where she's planning to play.

2 computers and the ps3? Why not just stay with one platform or the other..I would think it would be frustrating keeping the controls straight. And what about voice chat? I don't know if it works across both platforms.