Pregnancy Risk

A girl who I have been seeing for the past few months had sex Friday night. During intercourse, the condom broke and I finished inside of her (didn't know until afterwards). She immediately took Plan B. She is not on any BC. She said her cycle is 21-days. The last time she had her period/end of menstruation cycle was 15-days ago yesterday (16 today) and that they typically last for 5-days. She said she doesn't know when she ovulates. So if I did the math right, we had sex on Cycle Day 19. She is heavier than 165lbs. I've read that Plan B can be less effective for heavier women. How big of a risk is she for getting pregnant? I'm trying not to panick or worry her. Any advice would be helpful.