Can a former employer provide a useful recommendation for graduate admission to a science program?
The important thing to keep in mind is that the goal isn't this specific letter, but composing the best possible application for, in this case, Stanford. So the primary question is: would a letter from your supervisor at Apple improve your set of recommendation letters? If you already have 6 letters from very qualified people that know your work well and can comment on your ability to perform at the Stanford program, a letter from your former boss might not add much. However if you only have 3 people at the moment who can really say something about you, adding your former boss couldn't really hurt. A few things in mind are:


How long, how long ago and how closely did you work with your former boss compared to the other people writing you letter of recommendation?  
Was there a (strict) selection process to get into Apple? If so, the selection people at Stanford might be interested why the selection people at Apple chose you out of all possible candidates.  
Did you do any research at Apple, or answer any research questions? Even when not doing theoretical physics, your attitude towards discovery and finding new knowledge might be relevant.