Not quite the same but I feel pregnant after dtd at the end of AF still light spotting, o'd mid cycle as usual only dtd once as boyfriend at the time went away for work, missed my AF. I was pregnant apparently sperm lived for 10days. I was shocked but OB confirmed that it can happen. So I am guessing anything is possible.
Good luck, try to not worry to much and if you are tell her straight away and how it happened. Hopefully she understands.