if your friend was not able to back up his/her so called 'fact', then I'd doubt it is of much truth (unless he was thinking of cruciferous vegetables and the goitrogens (sp?) in them that block iodine assimilation?)
I've kind of just learnt to let these kinds of things people say (who haven't spent as much as an hour of their life reading about health) just slip in one ear and slip out the other one just as quickly... I'll give them an 'oh, really?' just to give them that benefit of a doubt, and I'll even research it if it sounds like it could be true or whatever. But most of the time, in my mind I'm just shaking my head and saying 'if only you knew...'
often times I hold back on telling people what I'VE read/heard/seen because I know the type of reaction I'll get from the sheeple, so I save myself the embarassment.