Am I sinful for ordering my friend to break her fast during the day in Ramadan when she was tired and had low blood pressure, fearing that she might faint, and what is the expiation for that, if it is due?

If your friend is ill with a sickness that makes fasting difficult for her, or if she fears the illness will worsen, then she is permitted to break her fast. She must make up the fast of that day later, as Allah Almighty says: "So whoever among you sights the New Moon of the month (of Ramadan), he must observe Saum (fasts) that month, and whoever is ill or on a journey, then (he should fast) a number of other days." There is no sin upon you for advising her of this.