@sirflimflam:
My guess is that the goddess allowed his soul to meet the soul of his dead grandpa before grandpa's soul went through the process of forgetting his life.
So the goddess temporarily brought the MC's soul [= the soul of a living being] to the place where his dead grandpa's soul [= the realm of the dead].
As already mentioned that is not a place a living being should be able to enter.
Hence she broke the rule that the living beings (or just their souls) shall not meet the the dead people (even just their souls).
As for the person who just incarnated into the world I suspect that that person is actually the goddess - now former goddess in that case.
Since she broke the rules of the gods (as mentioned above) the goddess was probably judged by whatever gods are in charge of controlling and enforcing the laws of the gods.
Her punishment is probably something like having to give up her position as goddess temporarily by living one lifetime as a human or humanoid race (whatever that person is).
Those are my assumptions.