Take the laptop away from her. She is too young to be able to surf the net like that alone. She didn't "accidently" discover a porn site either. She could also be going onto chat sites that are frequented by sexual predators. Think about that for a second. Do you really want her to be that "free" to stumble onto those types of things? I don't think so. Who's the parent - you or her?
Just because dad gave her this laptop does not make it right. Make her use the desktop computer in the family room/livingroom in your presence. Talk with her about her day, etc. And listen to what she says. Sounds like there is a lack of communication here. It's like plopping a toddler in front of a tv with a remote control and then leaving the room and coming back only to find he stumbled onto the Porn channel. So who's fault is that? The parent's fault.