OSX adware

Hi everyone. I'm supporting a friend who's on a Mac. I'm not sure what he stumbled into, but he's infected his computer with Adware. I've run Trend Micro's Office Scan on the box, which tells me the system is clean.

I've got a heckuvalot more experience supporting the Windows/PC world than I do Mac, so please bear with me.

How do you diagnose malware/adware variants on a Mac? Are there any good tools you can recommend? I can't clean the system until I can identify exactly what he's been hit with.

Mostly advertisements are appear on the Google home page. I've attached a few samples.

Many thanks in advance for the assistance.