I think that when Lester comes to camp for the second time and Dionysus sees him he is panicking inside because this is Apollo, his big brother who has always been there for him. To see him so stripped of his power and mortal is scary. Dionysus knows Zeus goes hard in Apollo, but I don’t think he realizes the full extent of the abuse until the trials. He also knows what it it like to be mortal, because he was a demigod before ascending, but Apollo’s form is even weaker than Dionysus had ever been because he is not even half god. Dionysus also knows a lot about loss, as seen by his way of dealing with it by trying not to become attached to demigods with their short life expectancies, and he is afraid that he will lose Apollo, who has literally been around since before he was born.
Ok so I got spoiled for Jason’s death a while ago and I’m only on The Dark Prophecy rn and the scene w Apollo drowning Commodus slapped so I had to make this twas a contractual obligation