When to tell someone what you really think?

I have a few co-workers that have become really good friends of mine. We work in different departments but I have manage to make friends with them. Every Friday I get invited out to lunch by them. Now they aren't friends with each other. But I am friends with all of them.

Today, one of them which is the Packaging Engineer, called me to go out and have lunch with her. She had just gotten back from her business/pleasure trip to Italy, and was very excited to tell me all about her trip. Last week I had already scheduled a lunch with my other friend who is Peruvian and works as an expeditor.

I wanted to hear all about the trip, so how do I tell the Peruvian I wont be going with her without hurting her feelings. I couldn't tell her I wasn't going anywhere, because she would have seen me on the elevators going with the Engineer. By the way to get to the Engineer's office I have to pass by the Peruvian's office. So, what do I do now?
As I just sat there thinking of something to tell her, she calls me and tells me she has to catch up on a lot of work. And that she had to cancel on me. I told her how dissapointed I was, and how I was looking forward to our lunch date. I basically made her feel bad.
All along I had a big smile...so, the Engineer and I went to lunch.

She started to tell me all about her trip, and how much fun she had had. She brought pictures and was very excited, I don't think I have ever seen her this excited. So, I just listened, she took over the whole conversation. All I could sneak in was Wow, really, how wonderful. I am very expressive with my gestures and big eyes. So, I do a lot of expression with my eyes. There were moments I had no need to say anything...she just looked at me and kept on going. She talked about the trip, about how she wants to become an actress, about her losing weight, etc. etc she bombarded me with so much information...and all I could say was, aha, wow....etc.

A few weeks before she left, she came over to my house to drop some things to me. And she introduced me to her boyfriend. (Have you ever met someone, barely had anything to say. But know that they are not the right person for them.) There were no reasons, I had barely met this guy. But my insticts told me he wasnt good for her. I usually get these feelings. Sometimes we meet people and I tell Dragon, I don't like them. She of course gets surprised but she knows about my extra senses...and she knows, I am right. (most of the time)

My friends tells me I am going to break up with my boyfriend today...I said oh really. I didn't like him for you anyway. I think he is totally wrong for you. When I met him I thought he wasnt right for you. And she looks at me and tells me; well, thanks for letting me know. (In my mind I thought oh no...why in the world did I just say those things?) There was a reason why I wasn't saying much.
At the time I met him, I didn't tell her anything. I just kept it to myself, and thought we will have a chance to talk and I will tell her what I thought about him. But how do I tell someone hey I think your boyfriend it totally wrong for you?

So, basically I have given her more of a motivation to leave this guy, and I don't really know him. He hasn't done anything to me so, why did I just tell her all of that?
She said I knew I had to do this, and with you telling me what you felt I know now I am doing the right thing. "Great" me and my big mouth. Oh well, it's probably for the best. I do trust my insticts, I haven't been let down by them. So, she probably is better off. I am waiting for her to call me and give me the details. Oh drama, oh me oh my!