To my knowledge, there are no known short or long term side effects of vaping DMT. As for ayahuasca, there have been studies done on the large populations of regular ayahuasca users in South America which showed no brain damage or abnormalities attributable to ayahuasca use.
Be thankful, I suppose, that your boyfriend is interested in this particular substance, it's probably the physically safest psychedelic on the planet.
Subjectively speaking, the harsh smoke can give me lung trouble; but that's that's the full extent of physical problems. I feel like
even pot has a greater long-term effect on my brain.
However, the changes in your boyfriend's worldview may be difficult to deal with. DMT use is about far, far more than some sort of pretty or entertaining experience; it's almost some sort of alien technology connecting the user to other worlds, space, the cosmos, their true selves, God, etc. He may have trouble integrating his experiences with what he previously knew. Chances are he will gain knowledge which it will be impossible for him to share, and that even if he began to articulate it would sound crazy.
If he has good experiences with DMT, he could well ask you to try it, because that will be the only way for you to begin to comprehend what he is talking, what we are talking, about. If you are close and can talk about anything, chances are he will need you to be there to be understanding, for the ineffable mysteries of the universe will have been laid out before him, and you may be one of the only people he can really try to talk to about that.
Be open, be open-minded, and be ready! Your boyfriend is safe, but there could be other trials ahead.