It'll probably need a service by canon to get it out... not easy to open up a sealed unit without ruining it.

Perhaps I should have been less sarcy... when I said it isn't easy to open up a lens, I actually meant:

You will certainly ruin your lens, and invalidate any warranty on it. It could also probably knacker your camera, so don't do it. Ever.

Well it's a pretty old lens, it's with his film SLR.  It's pretty big hefty thing, metal.  I can't remember which model it is, but it's definitely a canon.  I'll just tell him to take it to a repair shop.

Thanks guys.

Update: it appears it's the new camera.  He bought a 40D recently and it's the lens with that.  I just told him to contact canon for a warrenty job or the place he bought it from.

Nah nah, I realised that, I was just quite flippant and assumed that people would realise - then realised maybe the internet isn't the place for sarcasm when people lean towards a have-a-go attitude!

I had problems with the 55-200mm mk2 Canon zoom lens when I had my 350D. It had dust lens behind the last element (closest to the sensor) but as far as I could see, it didnt make much difference. Maybe because of where it was.

Best to get it checked out. Since they are knockin out more and more cheap focal lengths they seem to get poorer in construction.