Method 1: If you're at their computer, on Windows open a command prompt and type ipconfig /all. On OS X, choose System Preferences from the Apple menu. Once that opens, select Network and choose Advanced. With the Wi-Fi tab selected, the MAC address is the last line of information.
Method 2: Although I've not been successful at getting the correct MAC address via this method, you can try for yourself.
On a Windows machine, open a command prompt. Ping the IP address of which you'd like to know the MAC address. After you ping, at the command prompt type arp -a and press enter. I've read online where folks have been successful with this method however, anytime I've tried, I only get my MAC address and not the MAC of the IP I sent a ping to.