7 days late

My best friend has been trying to get pregnant since August. I was so nervous to tell her when I got pregnant because mine was not planned and she was starting to worry that she would have trouble. 
She ended up reacting badly at first and then she came around. Anyways... She is now 7 or 8 days late for her period but has taken 3 tests over the week and has had them all come up negative. She said she is always regular so it is very weird that she is late...
We both think it would be so cool to be pregnant together... What are the chances that she is but just not showing up on a test and when should she go to the doctor if she still hasn't gotten her period?