I doubt it would be remotely dangerous.
I haven't done lab cultures, but a vaporizer must practically sterilize the oven contents. The temperatures commonly used for moderate to high temp vaping are in excess of temps used for common sterilization, so even if it's a short duration I expect the removal of life to be near complete. If it truly concerns you, grind it up and offer it to guests who enjoy combusting to partake of when they visit, certainly nothing will survive fire.
But more to the point, the odds are quite low that there would be significant transfer of contagious microorganisms from his hands to the bud then to your lungs via heated inhalation through a device. There's a much greater risk of contagion from shared mouthpieces, and that's common practice that nobody questions.
If there were a risk, it would be chemical. Just how dirty is this fellow? Did he have, say, diesel on his hands as he handled your cannabis? If so, I would be unhappy about that and throw it out. If not, I'd be more concerned that the plant was sprayed with pesticides, which is to say, not very.
Or if it just grosses you out, throw it out and next time you see him tell him that was some good shit right there. Certainly if you think he doesn't wash after wiping his ass I wouldn't want to hit it.
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