Pathetic, extremely convenient backstory, perfectly irrational as these stories so often set things up to be.
I'm not suggesting this and similar things never happen, but to have this woman seemingly deteriorate into a shell of her former self, willing to completely write off her own 'pride and joy' as the final straw, apparently having never discussed the implied affair with anyone, feels grotesque.
From one page we see how considerate and supportive she is of the family, an anchor doing her best for her son absent of the father's presence at home. We have her going so far as to check up on him at work, and either then or in the intervening years that pass, whatever she discovered during that time gnawed her down to the bone. Her partner's visage and voice had become such an integral part to her entire existence, her own son had lost his identity and any purpose to her beyond bearing a resemblance.
We don't see anything else of this family, or their relationship with each other. If we want to joke about yandere traits being hereditary, maybe his mom really had been so devoted to her husband that everything she did for her child, wasn't.
And being a silly comedy, maybe that was the intended takeaway. Probably not.