a question

one of my gf's called me last night, crying her eyes out because her
lame low life bf (my opinion of dirtbag) said he uses a rubber but she
found out he sometimes doesn'tt and has been lying about it. many words
later, including my suggesting she dump dirtbag, she wonders what are
the odds. and how many days after her menstrual (sp) is she most likely
to get preggers. i'm thinking it's about a week