Don't worry so much about it, who is lying and who is not. The importtant thing is that all you 3 people involved - you, your friend and the 3rd/unknown party - need to be treated by the doctor. At this point there is no point in blaming anybody for possibly cheating on the other. It is not the end of the world and need not be the end of the relationship. You two in a real relationship just need to agree not to see other people and to stay monogamous (one partner only). Just forgive and let bygones be bygones. Next time, make sure you all use protection against STDs (latex condoms).