My friend claims that his screen went black after opening an email and when it came back on there were many files on his computer that he didn't recognize. Is this possible?

Oh yes - this is very possible. If the email had embedded malware, it could very well have caused the black screen while it was installing itself. It sounds like it's time for your friend to get his machine scanned for malware and maybe update his firewall, virus protection and spyware protections - any maybe learn to be more careful about the emails he opens from unknown sources.