As we all know Rosekiller is into some kinky shit. The kind of shit that leaves Barty with bruises for ages. All their friends know this and don’t really care beyond making sure they never walk in on them having sex ever again, please god.
James, however, only gets introduced to this after he starts dating Regulus, who treats him like the most precious thing in the world. He can't understand a reality in which people wouldn't want to be treated like that.
And thus he comes to the conclusion that Evan must be abusing Barty, it's the only think that makes sense, no matter how many times Barty tells him he is the most enthusiastic participant. After all, when he asked Sirius what was going on at home, he refused to tell him he was being abused and made all the excuses for his parents. So he brings up these concerns to Regulus, who assures him Barty asked for it. James gasps and says that's a horrible thing to say, and Regulus looks into the camera like he's in the office.
Evan, indifferent as he is to what James thinks of him, refuses to help Barty correct this misunderstanding. It's James's own fault he won't take Barty at his word, and besides, he can see how James got to that conclusion. Which he tells him, to his face, after James finally talks to him about it thanks to Regulus and Barty's prompting. He then asks him to get out of his room, because he was about to go check if Xenophilius, The Weed Dealer of Hogwarts Castle, knows where to get roofies. It's almost Barty's birthday, after all.
James runs to Barty with this information, of course, and Barty just about strangles him, because he'd asked for that scene ages ago, and he'd asked Evan to wait a while so he wouldn't know it was about to happen. So thanks for the heads up, he guessed.















