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.