Late period.

Hi ladies! I have a few questions about periods. My friend is 4 days late on her period. She has sex but with a condom and never lets her boyfriend cum inside. She had been late before for about 8 days but of course this worries her. But because I wanted to help and don't really know about this can you guys help me out..

1. What is the latest you have been late? (Not including pregnancy.)

2. What are reasons to be late?

3. When should you start worrying and see test?

Please answer!