The blonde princess was nice and all, but Seiden is truly the best friend Diarmuid and Bezriche could have had because LOOK AT HIM. He’s actually DOING something about this family’s messed up relationships - especially by telling Jaina that he would take her away if she wanted that, but also that her father is more than just the person she’s seen so far.
I’ve had the personal experience of someone telling me things about my parent that I cannot imagine them doing, because all my life they’ve been depressed. It didn’t make me forgive them for what they’ve done to me, but I was at least more informed in how I treated them.
Jaina doesn’t have to forgive Diarmuid, but Seiden is right to tell her about the kind of person Diarmuid used to be before his world was turned upside down. She deserves to know, and to make choices based on the fuller picture, and that’s what Seiden is acknowledging and trusting she will do.