pc-infected-alert on safari

Hello support world! My brother in law has a iMac (sorry I'm not sure specs I can get them though). He attempted to get on the internet tonight and every time he opened safari he received a pop up (will attempt to attach photo he sent me). The popup is from pc-infected-alert (.com) and states microsoft detected security error due to suspicious activity. Please contact certified live tech for help. Gives a phone number and states his system is infected with a Trojan. The actual safari window has a image of the microsoft security castle and a phone number to call along with what appears to be a scan report of the system.

Now I doubt his computer is actually infected. I wouldn't be surprised if his kids got into or clicked on something and hence the issue. Any help would be appreciated. I was going to attempt to dial in and work on it remotely however he is unable to click around the message to get a working safari browser. Any assistance would be appreciated.

iMac 27 inch (mid 2011)

OS X 10.9.5

Safari info will not allow access with this message up all the options under the safari menu bar item are grayed out except services, hide safari, hide other and quit safari.

thanks