The employee was off work and should not be expected to respond to work-related messages outside of business hours. Salaried positions do not equate to being on call at all hours. The boss's expectation is unreasonable and infringes upon the employee's personal time.
While the employee is not obligated to respond immediately, ignoring a boss's message can be seen as unprofessional. The boss was simply checking in, and a simple acknowledgement would have sufficed. The employee's dramatic reaction suggests a lack of commitment to their job.
The expectation of 24/7 availability is an unreasonable encroachment on an employee's personal time. While professional courtesy is important, employees are entitled to disconnect outside of working hours. The boss's inquiry, while seemingly innocuous, sets a precedent for constant accessibility, which is detrimental to work-life balance. However, a brief acknowledgment of the message could have mitigated the situation. But the boss is primarily at fault for blurring the boundaries between work and personal life.