He could reply to them, or be silent. I don't know why it's so deranged to "take your toys and go home" aka stop using a service (even loudly), if you find that a change has been made that is a dealbreaker for you.

The defense that you can only half-participate in g+ is not much of a defense because 1) people may not want to support changes of that sort with their patronage lest it continue as a trend, and/or 2) the thing being objected to was a forced escalation of participation - why would a user assume this would be the last and that half-participation in g+ will remain an option?

Perhaps observing this phenomenon should lead him to consider that there might be some substance to these critiques, and not just dismiss them all out of hand as some sort of "derangement".