Ill give you an example of a recent situation in my workplace, OP.
Someone high up in the company went on sick leave due to stress. We all had to work overtime to cover the work that they were leaving behind.
3 weeks into their sick leave the person with stress posted a status on facebook, with pictures, gleefully bragging about what a wonderful time they had in Barcelona for their weeks holiday.
Occupational health encourage any activity, including foreign holidays and drinks with friends, that will help with their recovery.
The rest of us were left with a rather hefty workload while she was off sunning herself. It left a bitter taste in the mouth of many other employees who finished work late and had to start early for a total of 12 weeks.
Then she resigned anyway.
While the employer legally doesn't have a leg to stand on in this case you are likely to make enemies of the people you worked with.
I really hope this helps.
I think this lays out nicely, why people do feel resentful when people go off work with stress. As you say, the Employer doesn't have a leg to stand on, because the person with stress is often encouraged to go out walking a lot, join the gym, even go on holiday
...... but it's still not nice for the people that are having to work all the hours going
Posting pics of your holiday on FB and going to the work's Do is one step too far.