So what was the point of telling that sob story to Sasae, does he just lie for fun?
Well, keep in mind that if this tree
is just Sasae 2's memories, Sasae 1's whole conversation with Dad at the table is a bit weird. Not only is it a conversation the kid shouldn't have heard, but also the dad claims it's information he's never told anyone, which would include Sasae 2, in which case how is it in Sasae 2's memories? Unless Dad randomly goes on a tangent about it after raping Manako or something that we just haven't seen yet, the memory shouldn't have that info to give to Sasae 1.
So with that peculiarity in mind, a not-uncommon trope in "memoryscape flashback spell" scenes like this is that powerful wizards are able to sense when someone's interacting with a memory/past version of themselves, basically jumping into the spell themselves a bit, through some aspect of the "real past" being in the spell or whatever (hence being able to interact with things outside of the scope of the real memory).
So it could be that Dad's been onto Sasae 2 from the get-go, and was intentionally lying because he kind of knows what's happening, and that Sasae 2 / "Kazan" is an adversary and they're in a spell, not just some random guy from the street.