^Tinnitus can be caused by effing anything in life. From what I've read, doctors don't know why people get it, nor do they have any solutions for it. Sometimes it just happens to people... for life. My girlfriend and her father both have tinnitus, and all they do is ignore it. That's all they can do. They've both had it for years, and it did not coincide with any specific event for either of them, just popped up out of nowhere one day.
For your friend, it might go away, might not. Does he stay hydrated? I've found when I get a bad ringing in my ear, I'm usually very dehydrated. I've also found that the majority of people I know (and probably Americans in general) don't hydrate enough, so that would be the first thing I would check. I've never personally had any ear-ringing issues with MXE over my 100+ uses, so somehow I doubt your friend's issue is due simply to a single time use of MXE.