I've never heard of a virus or spyware program doing that before. I've heard of people getting their passwords stolen in some instances, but I've never heard about any instance of someone installing an account on someone else's computer.
Spyware is usually just a snoop program that keeps an eye on what you're doing, and although viruses do a lot of things it usually involves screwing up the computer's sensitive data in some aspect. Sensitive data, unfortunately, usually isn't exactly somebody's IM account.
Although it is possible that he could have gotten a trojan horse type of virus that would have allowed someone to access his computer in the background, why would they install a Yahoo message account? Why wouldn't they go after any financial-type documents, or delete sensitive files, or something of the sort that would really cause havok? And how would they know that placing an account of that name on your computer would cause such a reaction, since nearly 100% of hackers don't know who their victims are?
Sounds like he's making excuses to me. Tell him to delete the account off your computer if he's so sure that it's not his. And if he does, check on the computer later to see if the 'mysterious' screen name returns.
But hey, that's just one man's opinion.