Dr. Lena Khan, a general practitioner in Vancouver, has been treating Mr. David Chen, a 55-year-old software engineer, for chronic headaches for several years. During a routine follow-up appointment, Mr. Chen enthusiastically describes a new tech startup he is launching and mentions he is seeking private investors. Dr. Khan, interested in the venture, expresses her desire to invest a significant sum. Mr. Chen, flattered and feeling a strong sense of trust due to their long-standing therapeutic relationship, agrees. Six months later, the startup faces severe financial difficulties, and Dr. Khan loses her investment. Feeling personally aggrieved and frustrated, Dr. Khan raises her concerns directly with Mr. Chen during a subsequent medical appointment, questioning his business acumen and suggesting his health issues might be impacting his judgment. Mr. Chen, feeling pressured and exploited, eventually files a complaint with the College of Physicians and Surgeons of British Columbia. Which of the following principles of medical ethics or professional conduct has Dr. Khan most significantly breached in this scenario?