How can I add apps to a locked iPhone?

So my friend Cal took off to Chile to go backpacking or something. Gave me his iPhone and charger, told me to go crazy with it til he gets back. I slipped the SIM card out of my LG Shine & stuck it in, and it worked GREAT. That is, until I needed to download apps. They say I need his password, even though I have the iPhone hooked up to my computer and my iTunes. How do I switch the iTunes accounts? Stupid Cal hasn’t been on facebook for 2 weeks so I’ve given up on tyring to reach him. You guys are my last hope.. =)