I'm sorry for posting this, since it might be bleeding obvious for you good people.

I have someone interested in buying my Nikkor 28-70mm f/2.8, but they have asked me a question that I simply do not know the answer to. He has a Nikon D200. Will the lens limit the speedlight in any way (regarding functionality, ttl metering, etc.) on this camera? Being the pro lens it is, my impression is no - but I haven't used a flash for 20 years, so...

He says he has a Tokina 28-70 f/2.8 that causes the flash to fire at full force no matter what.

I always thought that flash functonality was a result of the camera and flash itself, and nothing to do with the lens - but obviously I have some reading up to do here

If he ends up buying your lens he may discover that his Tokina may not have been fully communicating with his camera and speedlight.

Whenever you buy a 3rd party lens, there's always the liklihood that it may not be fully recognized by the camera body. The D200 does have a menu item that allows you to identify some of the physical parameters for lenses it fails to identify, but I'm not sure if that information is passed back to the flash. This is less of an issue with newer 3rd party lenses these days, but depending on how old his Tokina is that may well be what the issue is.

Theoretically there should be no issues, but If I were you I would certainly not make any guarantees. If he is having issues with his D200, you couldn't be held responsible.