It hadn't exactly been a surprise to Five that they had all gone their seperate ways when he returned from the future but he had fought to get back and save them all, no matter what so for him it was clear that helping Vanya was what they should all do. They were family, even if they didn't want to be.
"We will because you deserve to live your life without those pills" Five insisted, "It's a lot I know, and I'm sorry we kept it from you" Even if he had wanted to tell her, he'd made the choice not to at the time even though he had felt it was wrong.