Stable income: Somewhat stable

Mystery shopping can be a viable side hustle for some, but it's important to note that the income can be inconsistent. While some individuals may earn steadily, others experience fluctuations in their earnings. Based on online feedback and personal observations, it appears that most people do not earn consistently each month in this field. Therefore, if your best friend quit their job to pursue mystery shopping full-time, I would say it's a risk rather than a reliable source of income.