Only if you pretend that the surrounding context doesn't exist. If you don't, then it should be pretty obvious that this is just the most recent in a long line of signs and red flags that the guy either didn't notice or ignored. There's no way that you get to the point in a relationship of having a kid with someone and only at that point get blindsided by them with something like this from out of the clear blue sky.
This was a long, twisted, passive-aggressive, denial-filled, and resentment-fueled path that led those two to this spot on the reddit map of fucked up-ness. Cutting out the context and just thinking about what you'd do about this single thing in this isolated, context-less moment is silly.
IMO, the real question is: How did this guy wind up here?