Your feelings ARE important. You were told verbally it was okay and it wasn't, and now you have this guilt hanging over your head for something that isn't your fault. I've been in this situation multiple times and it SUCKS. Just remember that their relationship is not your responsibility, and there's nothing wrong with having trusted your long time friend to have not lied to you. It was not your responsibility to figure out how Kate was feeling, it was HIS relationship and HIS responsibility. You did nothing wrong. I know it will feel really shitty, but I recommend trying to separate yourself from them as much as possible until they figure things out. Usually, you are painted as the "villain" in this scenario even though you didn't technically do anything wrong, and you shouldn't have that hanging over your head. They probably also need to sort out what happened for now. However, try your best to keep yourself occupied and hang out with other friends abd immerse yourself in hobbies or whatever can distract you. The weight of their relationship is not on your shoulders, the guy made his decisions and I'm sure he realized how she would have felt.