If I were in charge of the Percy Jackson sea of monsters show I would make a really sad scene where Poison Tree by Grouper is playing and Luke is having flashbacks of him and Thalia back in the day and he’s having a whole moral struggle with having to poison Thalias pine tree and I would make it as sad as possible I would make Luke cry tears running down his entire face the whole time he’s poisoning her tree and having flashbacks of them being happy together and then having fights too. A “we used to to be best friends forever where did it all go wrong” type of moment.
Also maybe have him whisper something about the Golden Fleece at the end. Trying to reassure himself that Thalias tree won’t die. Reassuring himself that either him or someone in the titan army or the campers will find the Golden Fleece and save her with it in time. He’ll get to see Thalia again someday.