As of right now, it's hardly a spoiler to say that Obi-Wan Kenobi will feature the title character's first meeting with Anakin Skywalker since Revenge of the Sith. The fact they had a rematch a decade before the events of A New Hope is one of this show's big selling points, and we've seen that teased both in the teaser trailer and some recently revealed promo art.
This would explain why "Ben" wasn't shocked by his former Padawan's appearance in Episode IV, of course, but how will the Jedi Master react to seeing what's become of Anakin? The last time they crossed paths, Obi-Wan left Anakin to burn for his role in destroying the Jedi Order, though we have to assume he somehow became aware that Emperor Palpatine "saved" him.
Either way, Making Star Wars has shared some details on what happens when Obi-Wan is reunited with Vader, explaining that "Most everyone paraphrased the moment as Obi-Wan feeling slightly appalled and muttering something like 'Oh my god, what have they done to you? I’m so sorry.' He feels shocked and he feels sympathy for the sorry state of Anakin Skywalker."
The site goes on to remind us that we'll see the horrors of how Vader stars his day at some point in Obi-Wan Kenobi when he emerges from his Bacta Tank and is reassembled by droids. It's said that the point of all this is to remind us that while Vader and Palpatine "won" in Revenge of the Sith by toppling the Jedi Order, Obi-Wan's exile pales in comparison to what Vader goes through.
In other words, we'll learn that Vader isn't your typical villain, enjoying the fruits of his victory. It's also interesting to see that Obi-Wan may express remorse at his role in Anakin's horrific transformation.
More will be revealed when Obi-Wan Kenobi premieres with its first two episodes on May 27.