I have a brewing friend who is a bit crazy on unorthadox adjuncts and he wants to make a tabacco flavoured stout or heavy ale.. I know there are some spirits infused with tabacco, but probably not any beer so far.

Does anyone have any idea how this would be approached, or how much could be used before it became poisonous?

Yes, now that you've mentioned it should be aged I have realised I will probably have to look into it a lot more before I just chuck anything in there. My brightest idea so far is to crumble up a cigar and soak it in rum in a muslin bag and then leave that in the secondary for 10 days: like you can do with oak chips. I'm getting excited about the idea now, even though I discouraged my friend at first. It's probably because I stopped smoking a while ago and this is a kind of camoflaged relapse.