First off, when the family was lamenting Jim' stroke, John's brain was blank. April wanted her grandpa back, Elly and Phil wanted their dad back, Iris wanted her husband back and John....probably wanted to get a bite to eat but was hampered by the sad people talking about the old dude who just upped and stroked out like old people did.
We also have to remember that John fears having to contemplate his own mortality. We have a rejected strip that has him wish to ignore the problem because it's too depressing and a later strip that suggests that if Elly becomes a burden, he might simply smother her for the insurance money.
Finally, we have to remember the fact that John has never sat down and actually talked to Jim about anything that I've noticed. He might have been under his son-in-law's roof for a while but they never compared notes because John feared being told off by someone who could back up his discontent.
This leads us to the inevitable hypothesis as regards how John sees his father-in-law's stroke: "What happened to Elly's dad is a shame but not something that should affect me as long as he has people taking care of him."