Should I have just gone on my family vacation and not have covered the shift?

I worked in retail for a handful of years, and during that time, I have made great lifelong friends. Because I am a hardworking, committed employee, my boss/owner took notice and she promoted me to manager at age 17. My fifth year working at the store, we hired a new employee, Lindsey, and we instantly became great friends. We became so close that we were hanging outside of work, and Lindsey introduced me to her group of friends. My new group of friends and I became super close, where I would see them every day. Lindsey had a lot of health problems, and she would frequently call out sick. Being a manager, I had to take responsibility and cover her shifts, even though I would have other obligations. I requested the Fourth of July weekend off to go to my lake house with my family. At the last minute, however, Lindsey called out sick, and I received a phone call from my boss begging me to cover her shifts, which I did. On July 3, my friend Rob invited me to come over to his family BBQ, because he knew I was missing my family BBQ. He told me all of our friends would be there as well, so I planned to head over there after I got out of work. I worked ten hours that day, I was exhausted, but I wanted to see my friends. I pulled up to his house, and I saw Lindsey’s car there, I was extremely confused because Lindsey called out sick for the whole weekend, so why was she there? Lindsey did not seem the least bit sick; in fact, she was socializing and having the time of her life. I could not believe Lindsey just put me in this horrible position—I missed my family vacation because she did not “feel” like working that weekend. If I ratted her out, I would lose the new friends with whom I had become extremely close. As her manager, should I tell my boss that Lindsey was faking it, even though we are friends outside of work? Where does the line end between friends and employees?