When I was training for a race this winter, I would just let people know MY ride plans - "I'm riding at 10am from X location, pace will be xx mph, and I'm going for 80 miles. Let me know if you can make it." That way, I get in exactly what I need, and if anyone wants to show up, they can.

If your friend has planned a ride that doesn't fit your schedule, you simply can tell her that doesn't fit into your training program.