Merely because your relationship with your boss is professional by definition does not mean that you are prohibited from taking an appropriate personal interest in her well-being. For example, if one of your boss's loved ones dies, you certainly can extend your sympathy and concern through a card. But there are a number of considerations to keep in mind when writing a sympathy card to a boss.
In expressing written sympathy to your boss, a crucial preliminary step is that you select an appropriate card. The selection of sympathy cards available on the market necessarily, however, is rather limited. The two major classification of cards are religious and more secular.
Because of the nature of the workplace, the appropriate choice is to select a sympathy card that does not have a religious theme. There are exceptions to this suggestion (e.g., if you attend the same place of worship as your boss). However, in order to avoid any suggestion of impropriety, stick with a basic sympathy card lacking religious themes.
An often used directive in a professional or workplace setting is "keep it simple." The same holds true when writing a sympathy card for your boss. Do not draft anything lengthy or elaborate. Convey directly and honestly your sympathy for your boss's loss.
The fact that your boss lost someone important in his life does not lift the professional boundaries that exist in the workplace. While you do not need to sound like you are dictating a work-related memo, you must avoid an intimacy that does not exist and suggests a closeness that generally is not appropriate between a boss and an employee.
If you happen to know the deceased (for example, if your boss's spouse died), you appropriately should make some specific comments about the individual. Again, do not be elaborate or lengthy. Write a remark about a particular attribute that you admired in the person who passed on, but avoid listing every good quality. Simplicity and sincerity are most appropriate in this context.