Definitely vent a buddy heater to be prudent. With that said...
I played around with a buddy heater or 2 at work the past few weeks. The co output was almost below the detection limit of my equipment (1ppm), we have WAY more issues running into co issues with our Honda 2k generators. Running 10' or so away from wall tents, we easily surpass safe levels very quickly if the wind blows wrong.
I use a buddy heater, I also have a co detector in the area. I sleep well, and warm.