We never actually get to see Webber read his letter from Karev because he tosses the paper into the trash before his AA meeting. But he does reveal the contents to his group, explaining that while reading the letter, he initially wanted to throttle Karev. He was mad at him for abandoning all the work he's put into at Grey Sloan (16 years worth of work!) and the beginning of their renovations at Pac-North. But he notes that what makes him truly upset is that Karev said goodbye with a letter, robbing Webber of the chance to say anything in person and close their chapter properly. He openly mourns the loss of a former student, a peer, and a friend, noting that it feels like he's losing everyone important to him.