What is the ruling regarding someone who performed ghusl after menstruation and intended to fast, then discovered a small amount of blood descending during the day of that fast? And does she have to make up two days (one day for menstruation and one day for eating) or only one day?

Your friend's words are incorrect. As long as you have seen menstrual blood again, you have become a menstruating woman, and it is obligatory for you to stop fasting, praying, and all other things that a menstruating woman abstains from. Your previous intention holds no weight. What you did by breaking the fast on that day was correct, and you are only required to make up one day.