How did she manage to see his dick? WHY in the world would she tell you? This doesn't sound kosher. Maybe she wants a threesome and is trying to test you.
A friend of mine got married a long time ago and he and his new wife moved to London to a bedsit (flat) with shared bathroom, they were having a bath when a guy from another flat walked in nude, (they had forgotten to lock the door) anyway accidently seeing his cock made an impression on her, two days later when my mate came home from work she was in bed with the guy. marriage over.