Is it true that people with emphysema shouldn't breathe 100% oxygen?

Yes and No: Patients with emphysema are at some risk for developing high carbon dioxide levels (hypercapnea) if given high doses of oxygen. The oxygen will displace the carbon dioxide off of the red blood cell resulting in higher blood levels (haldane effect). However, hypoxia (low oxygen) is more likely to kill a patient than hypercapnea (high co2) so using high dose oxygen may be needed.

Sort of: In a small proportion of patients with emphysema, the lungs are so damaged that the body actually depends on the low oxygen levels to drive breathing. In these patients, giving high amounts of oxygen can actually be harmful, as it eliminates the drive to breathe and can actually cause breathing failure. But, it is hard to know who these patients are.