First, I don't know much about computer technicals:

however, right now I'm looking at my "DHCP Client List" for my network (just a wireless router for the home) and it's showing a computer that was here earlier but is no longer here.... now the MAC address shows up even if I refresh... I'll shut down other computers off and they go off the DHCP Client List so I know it's refreshing... so why does my friend's computer still show up on my network???

P.S. I also made a bet with this friend as a joke that she couldn't get onto my network. 2 minutes on one of my laptops and she was reading to me my own email addresses and passwords, my network passwords and names, bank site passwords and logins... etc.... Apple, btw, are they that easy to hack?

But the main question is why is her laptop on my Client List even though she's not here nor is her computer?

Hard to say.

Guess it depends on what brand of router you use. Many routers tend to store/remember MAC addresses, and whenever that particular PC/laptop appears again it will hand out the same IP address as before. I know my D-link does that.

Unplugging from the power doesn't seem to clear that either.

With my router, that MAC address only disappears when it runs out of IP addresses to assign.

What she probably did was sniff the local machines on the net - locate yours, and then spoof your MAC address. This is a clessic example of why wireless connectivity is so insecure. 99% of users don't have a clue on these hacks, but one-percent do. I have no idea what the configuration of your system is so I can't advise on a cure. But the lesson here is; There is no security system that can't be overcome.

What she probably did was sniff the local machines on the net - locate yours, and then spoof your MAC address. This is a clessic example of why wireless connectivity is so insecure. 99% of users don't have a clue on these hacks, but one-percent do. I have no idea what the configuration of your system is so I can't advise on a cure. But the lesson here is; There is no security system that can't be overcome.

The OP never stated that his MAC address was spoofed, he just said that a MAC address of a computer which is note even there shows up.

So either his router is not letting go of the IP address lease, or the friend is sitting outside his place still connected!